auth: Change the look of SOCIAL_AUTH_SUBDOMAIN when directly opened.

SOCIAL_AUTH_SUBDOMAIN was potentially very confusing when opened by a
user, as it had various Login/Signup buttons as if there was a realm on
it. Instead, we want to display a more informative page to the user
telling them they shouldn't even be there. If possible, we just redirect
them to the realm they most likely came from.
To make this possible, we have to exclude the subdomain from
ROOT_SUBDOMAIN_ALIASES - so that we can give it special behavior.

zerver/middleware.py

@@ -576,7 +576,10 @@ class HostDomainMiddleware(MiddlewareMixin):
             return None
 
         subdomain = get_subdomain(request)
-        if subdomain != Realm.SUBDOMAIN_FOR_ROOT_DOMAIN:
+        if (
+            subdomain != Realm.SUBDOMAIN_FOR_ROOT_DOMAIN
+            and subdomain != settings.SOCIAL_AUTH_SUBDOMAIN
+        ):
             request_notes = RequestNotes.get_notes(request)
             try:
                 request_notes.realm = get_realm(subdomain)