WIP: Fix hanging registration when background modes disabled

fixes #1329

We must register notification settings earlier to be sure we'll be able
to get the users push tokens.

We were previously relying on a quirk of background fetch, a feature
which we don't *actually* use, wherein it's possible to get push tokens
for a user without first prompting them for their notification
settings. More on this from UIApplication.h:

    // Calling this will result in either application:didRegisterForRemoteNotificationsWithDeviceToken: or application:didFailToRegisterForRemoteNotificationsWithError: to be called on the application delegate.
    // ---> Note: these callbacks will be made only if the application has successfully registered for user notifications with registerUserNotificationSettings:, or if it is enabled for Background App Refresh.
    - (void)registerForRemoteNotifications NS_AVAILABLE_IOS(8_0);

In the previous implementation, if a user had disabled background
notifications (e.g.  in order to save battery), we were waiting for push
tokens that would never come.

However, this simple "fix" introduces a UX problem - now we prompt for
an alert in the middle of our registration flow.

// FREEBIE

Signal/src/view controllers/CodeVerificationViewController.m

@@ -100,8 +100,6 @@
 
 - (void)setupContacts {
     [[Environment getCurrent].contactsManager doAfterEnvironmentInitSetup];
-
-    [[PushManager sharedManager] validateUserNotificationSettings];
 }
 
 - (NSString *)validationCodeFromTextField {
@@ -110,7 +108,8 @@
 
 - (TOCFuture *)pushRegistration {
     TOCFutureSource *pushAndRegisterFuture = [[TOCFutureSource alloc] init];
-    ;
+
+    [[PushManager sharedManager] validateUserNotificationSettings];
     [[PushManager sharedManager] requestPushTokenWithSuccess:^(NSString *pushToken, NSString *voipToken) {
       NSMutableArray *pushTokens = [NSMutableArray arrayWithObject:pushToken];