ios notifications: Update the badge count if the app is foregrounded.

This is based on the recommended setup for PushNotificationIOS from
React Native [1], so we might have included it in the next commit,
where we fully set that up. But as Greg points out [2], this code is
purely using upstream iOS APIs, so it doesn't depend on whether
we're using that RN module or something else, so it's fine to do
here on its own.

In a departure from those instructions, we decide to just pass the
thing (`UNNotificationPresentationOptionBadge`) that updates the
badge count when the app is foregrounded. This is a necessary part
of zulip#4182, but one thing we'll still need to do for that is test that
the server is sending the right badge counts to the client.

[1] https://reactnative.dev/docs/0.62/pushnotificationios. These
    instructions are actually missing a required change to the
    AppDelegate.h, which we take here, having seen it in the
    instructions for @react-native-community/push-notification-ios
    (at
    https://github.com/react-native-push-notification-ios/push-notification-ios#update-appdelegateh).

[2] zulip#4163 (comment)

ios/ZulipMobile/AppDelegate.h

@@ -1,8 +1,9 @@
 #import <React/RCTBridgeDelegate.h>
 #import <UIKit/UIKit.h>
 #import <UMReactNativeAdapter/UMModuleRegistryAdapter.h>
+#import <UserNotifications/UNUserNotificationCenter.h>
 
-@interface AppDelegate : UIResponder <UIApplicationDelegate, RCTBridgeDelegate>
+@interface AppDelegate : UIResponder <UIApplicationDelegate, RCTBridgeDelegate, UNUserNotificationCenterDelegate>
 
 @property (nonatomic, strong) UMModuleRegistryAdapter *moduleRegistryAdapter;
 @property (nonatomic, strong) UIWindow *window;

ios/ZulipMobile/AppDelegate.m

@@ -57,6 +57,11 @@ - (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(
   rootViewController.view = rootView;
   self.window.rootViewController = rootViewController;
   [self.window makeKeyAndVisible];
+
+  // Define UNUserNotificationCenter
+  UNUserNotificationCenter *center = [UNUserNotificationCenter currentNotificationCenter];
+  center.delegate = self;
+
   return YES;
 }
 
@@ -120,4 +125,16 @@ - (void)application:(UIApplication *)application didReceiveLocalNotification:(UI
   [RNNotifications didReceiveLocalNotification:notification];
   [RCTPushNotificationManager didReceiveLocalNotification:notification];
 }
+
+// Called when a notification is delivered to a foreground app.
+-(void)userNotificationCenter:(UNUserNotificationCenter *)center
+      willPresentNotification:(UNNotification *)notification
+        withCompletionHandler:(void (^)(UNNotificationPresentationOptions options))completionHandler
+{
+  // Update the badge count. Do not play sound or show an alert. For
+  // these options see
+  // https://developer.apple.com/documentation/usernotifications/unnotificationpresentationoptions?language=objc
+  completionHandler(UNNotificationPresentationOptionBadge);
+}
+
 @end