validate push settings on main thread

// FREEBIE

Signal/src/Models/SyncPushTokensJob.swift

@@ -27,11 +27,13 @@ class SyncPushTokensJob: NSObject {
     func run() -> Promise<Void> {
         Logger.info("\(TAG) Starting.")
 
-        // Required to potentially prompt user for notifications settings
-        // before `requestPushTokens` will return.
-        self.pushManager.validateUserNotificationSettings()
+        let runPromise: Promise<Void> = DispatchQueue.main.promise {
+            // Required to potentially prompt user for notifications settings
+            // before `requestPushTokens` will return.
+            self.pushManager.validateUserNotificationSettings()
 
-        let runPromise: Promise<Void> = self.requestPushTokens().then { (pushToken: String, voipToken: String) in
+            return self.requestPushTokens()
+        }.then { (pushToken: String, voipToken: String) in
             var shouldUploadTokens = false
 
             if self.preferences.getPushToken() != pushToken || self.preferences.getVoipToken() != voipToken {

Signal/src/network/PushManager.m

@@ -290,6 +290,8 @@ NSString *const Signal_Message_MarkAsRead_Identifier = @"Signal_Message_MarkAsRe
 }
 
 - (void)requestPushTokenWithSuccess:(pushTokensSuccessBlock)success failure:(failedPushRegistrationBlock)failure {
+    AssertIsOnMainThread();
+
     if (!self.wantRemoteNotifications) {
         DDLogWarn(@"%@ Using fake push tokens", self.tag);
         success(@"fakePushToken", @"fakeVoipToken");