Add discarded_notification_center_observer rule

[marcelofabri]

Fixes #1062

This really should be a compiler warning because of the unused return value, but I guess this is not applied for methods imported from Objective-C. When using the OSS Foundation on Linux, a warning is generated.

[SwiftLintBot]

	18 Warnings
⚠️	This PR introduced a violation in ios-oss: /Kickstarter-iOS/Views/Controllers/RewardPledgeViewController.swift#L140:5: warning: Discarded Notification Center Observer Violation: When registing for a notification using a block, the opaque observer that is returned should be stored so it can be removed later. (discarded_notification_center_observer)
⚠️	This PR introduced a violation in ios-oss: /Kickstarter-iOS/AppDelegate.swift#L153:5: warning: Discarded Notification Center Observer Violation: When registing for a notification using a block, the opaque observer that is returned should be stored so it can be removed later. (discarded_notification_center_observer)
⚠️	This PR introduced a violation in ios-oss: /Kickstarter-iOS/Views/Controllers/ActivitiesViewController.swift#L20:5: warning: Discarded Notification Center Observer Violation: When registing for a notification using a block, the opaque observer that is returned should be stored so it can be removed later. (discarded_notification_center_observer)
⚠️	This PR introduced a violation in ios-oss: /Kickstarter-iOS/Views/Controllers/ActivitiesViewController.swift#L25:5: warning: Discarded Notification Center Observer Violation: When registing for a notification using a block, the opaque observer that is returned should be stored so it can be removed later. (discarded_notification_center_observer)
⚠️	This PR introduced a violation in ios-oss: /Kickstarter-iOS/Views/Controllers/CheckoutViewController.swift#L102:5: warning: Discarded Notification Center Observer Violation: When registing for a notification using a block, the opaque observer that is returned should be stored so it can be removed later. (discarded_notification_center_observer)
⚠️	This PR introduced a violation in ios-oss: /Kickstarter-iOS/Views/Controllers/CommentsViewController.swift#L30:5: warning: Discarded Notification Center Observer Violation: When registing for a notification using a block, the opaque observer that is returned should be stored so it can be removed later. (discarded_notification_center_observer)
⚠️	This PR introduced a violation in ios-oss: /Kickstarter-iOS/Views/Controllers/DiscoveryPageViewController.swift#L30:5: warning: Discarded Notification Center Observer Violation: When registing for a notification using a block, the opaque observer that is returned should be stored so it can be removed later. (discarded_notification_center_observer)
⚠️	This PR introduced a violation in ios-oss: /Kickstarter-iOS/Views/Controllers/DiscoveryPageViewController.swift#L35:5: warning: Discarded Notification Center Observer Violation: When registing for a notification using a block, the opaque observer that is returned should be stored so it can be removed later. (discarded_notification_center_observer)
⚠️	This PR introduced a violation in ios-oss: /Kickstarter-iOS/Views/Controllers/LiveStreamCountdownViewController.swift#L227:5: warning: Discarded Notification Center Observer Violation: When registing for a notification using a block, the opaque observer that is returned should be stored so it can be removed later. (discarded_notification_center_observer)
⚠️	This PR introduced a violation in ios-oss: /Kickstarter-iOS/AppDelegate.swift#L148:5: warning: Discarded Notification Center Observer Violation: When registing for a notification using a block, the opaque observer that is returned should be stored so it can be removed later. (discarded_notification_center_observer)
⚠️	This PR introduced a violation in ios-oss: /Kickstarter-iOS/Views/Controllers/LiveStreamContainerViewController.swift#L95:5: warning: Discarded Notification Center Observer Violation: When registing for a notification using a block, the opaque observer that is returned should be stored so it can be removed later. (discarded_notification_center_observer)
⚠️	This PR introduced a violation in ios-oss: /Kickstarter-iOS/Views/Controllers/LoginToutViewController.swift#L41:5: warning: Discarded Notification Center Observer Violation: When registing for a notification using a block, the opaque observer that is returned should be stored so it can be removed later. (discarded_notification_center_observer)
⚠️	This PR introduced a violation in ios-oss: /Kickstarter-iOS/Views/Controllers/ProjectNavBarViewController.swift#L51:5: warning: Discarded Notification Center Observer Violation: When registing for a notification using a block, the opaque observer that is returned should be stored so it can be removed later. (discarded_notification_center_observer)
⚠️	This PR introduced a violation in ios-oss: /Kickstarter-iOS/Views/Controllers/ProjectNavBarViewController.swift#L57:5: warning: Discarded Notification Center Observer Violation: When registing for a notification using a block, the opaque observer that is returned should be stored so it can be removed later. (discarded_notification_center_observer)
⚠️	This PR introduced a violation in ios-oss: /Kickstarter-iOS/Views/Controllers/RootTabBarViewController.swift#L14:5: warning: Discarded Notification Center Observer Violation: When registing for a notification using a block, the opaque observer that is returned should be stored so it can be removed later. (discarded_notification_center_observer)
⚠️	This PR introduced a violation in ios-oss: /Kickstarter-iOS/Views/Controllers/RootTabBarViewController.swift#L20:5: warning: Discarded Notification Center Observer Violation: When registing for a notification using a block, the opaque observer that is returned should be stored so it can be removed later. (discarded_notification_center_observer)
⚠️	This PR introduced a violation in ios-oss: /Kickstarter-iOS/Views/Controllers/RootTabBarViewController.swift#L26:5: warning: Discarded Notification Center Observer Violation: When registing for a notification using a block, the opaque observer that is returned should be stored so it can be removed later. (discarded_notification_center_observer)
⚠️	This PR introduced a violation in ios-oss: /Kickstarter-iOS/Views/Controllers/LiveStreamContainerViewController.swift#L90:5: warning: Discarded Notification Center Observer Violation: When registing for a notification using a block, the opaque observer that is returned should be stored so it can be removed later. (discarded_notification_center_observer)

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📖	Linting WordPress-iOS with this PR took 8.6s vs 10.39s on master (17% faster)
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[jpsim]

👍

FYI @mbrandonw, since Kickstarter seems to be pretty affected by this.