backend: Add support for mobile_flow_otp in social auth.

It turns out that very little code change is required to support
GitHub auth on mobile.  Ideally, this would come with tests, though
the complicated part of the code path is covered by the Google auth
version.  But writing a test for this would take a long time, and I
think it's worth having the feature now, so I'll be doing tests as a
follow-up project.

zproject/backends.py

@@ -229,10 +229,12 @@ class SocialAuthMixin(ZulipAuthMixin):
         is_signup = strategy.session_get('is_signup') == '1'
 
         subdomain = strategy.session_get('subdomain')
-        if not subdomain:
+        mobile_flow_otp = strategy.session_get('mobile_flow_otp')
+        if not subdomain or mobile_flow_otp is not None:
             return login_or_register_remote_user(request, email_address,
                                                  user_profile, full_name,
                                                  invalid_subdomain=bool(invalid_subdomain),
+                                                 mobile_flow_otp=mobile_flow_otp,
                                                  is_signup=is_signup)
         try:
             realm = Realm.objects.get(string_id=subdomain)