Move the media caption composer (and Add Caption action) behind a feature flag for now. (#3560)

ElementX/Sources/Application/AppSettings.swift

@@ -47,6 +47,7 @@ final class AppSettings {
         case fuzzyRoomListSearchEnabled
         case enableOnlySignedDeviceIsolationMode
         case knockingEnabled
+        case createMediaCaptionsEnabled
     }
     
     private static var suiteName: String = InfoPlistReader.main.appGroupIdentifier
@@ -280,6 +281,9 @@ final class AppSettings {
     @UserPreference(key: UserDefaultsKeys.knockingEnabled, defaultValue: false, storageType: .userDefaults(store))
     var knockingEnabled
     
+    @UserPreference(key: UserDefaultsKeys.createMediaCaptionsEnabled, defaultValue: false, storageType: .userDefaults(store))
+    var createMediaCaptionsEnabled
+    
     #endif
     
     // MARK: - Shared