Notifications PingHub

[jleandroperez]

Details:

• Create PingHub client Pinghub (Swift 3) #6347.
• Keep PingHub connected as long as possible. PingHub Manager, or how to stay connected #6357
• Improve notification syncing so we ensure we don't miss notes but don't duplicate sync if not needed.

Verify:

• Verify that Notification Actions (Edit Comment / Reply) result in a PingHub update message
• Verify that marking a Note as Read (in another device) results in a PingHub update message
• Verify that destructive actions result in "notification deleted" notification