ios [nfc]: Remove unneeded imports in ZulipMobile-Bridging-Header.h

This file, and these lines in it, were added early in the project's
history, in d38129b. At that commit, we didn't even have any Swift
files in our source tree, so it's unclear what benefit was sought by
adding this file, or why these particular lines were chosen.

Anyway, as mentioned in the code comment we add now, the lines had
the effect of making the named Objective-C modules available in our
Swift files without those Swift files needing an import line of
their own. They also made transitively imported modules available,
apparently including Foundation and UIKit.

With the lines removed, we need to add import lines in the Swift
files, following Xcode's complaints about what's missing. So, do
that.

Author: chrisbobbe

ios/ZulipMobile-Bridging-Header.h

@@ -1,8 +1,18 @@
-#import <React/RCTBridgeModule.h>
-#import <React/RCTBridge.h>
-#import <React/RCTEventDispatcher.h>
-#import <React/RCTRootView.h>
-#import <React/RCTUtils.h>
-#import <React/RCTConvert.h>
-#import <React/RCTEventEmitter.h>
-#import <React/RCTBundleURLProvider.h>
+// To use an Objective-C module in a Swift file, first just try importing it
+// at the top of your Swift file, like so:
+//
+//   import React.RCTBridgeModule
+//
+// If you can't find an import line that Xcode understands, instead try
+// adding an import line in this file, like so:
+//
+//   #import <React/RCTBridgeModule.h>
+//
+// That should make the module (plus the modules *it* imports, actually)
+// available in all our project's Swift files, without the Swift files
+// needing an import line of their own.
+//
+// The first approach (an import line in the Swift file) is preferred
+// because it looks like how imports normally work in Swift. But sometimes
+// we can't find an import line that works; not sure why. Discussion:
+//   https://chat.zulip.org/#narrow/stream/243-mobile-team/topic/ios.2FZulipMobile-Bridging-Header.2Eh/near/1520435

ios/ZulipMobile/ZLPConstants.swift

@@ -1,4 +1,5 @@
 import Foundation
+import UIKit
 
 @objc(ZLPConstants)
 class ZLPConstants: NSObject {

ios/ZulipMobile/ZLPNotifications.swift

@@ -1,3 +1,7 @@
+import Foundation
+import UIKit
+import React.RCTBridgeModule
+
 @objc(ZLPNotifications)
 class ZLPNotifications: NSObject {
   // For why we include this, see