auth: Fix redirect loop in development environment.

Since the REALMS_HAVE_SUBDOMAINS migration in development, we've had
scattered reports of users who found trying to open 127.0.0.1:9991
resulting in a redirect loop between zulipdev.com:9991,
zulipdev.com:9991/devlogin, and zulipdev.com:9991/devlogin/, and back
to zulipdev.com:9991.

We fix this temporarily through a small cleanup, which is to have that
last step in the loop send the user to the subdomain where they're
actually logged in, zulip.zulipdev.com:9991.

There's more to be done before this system will make sense, though.

zerver/views/auth.py

@@ -485,7 +485,7 @@ def redirect_to_misconfigured_ldap_notice(error_type):
 def login_page(request, **kwargs):
     # type: (HttpRequest, **Any) -> HttpResponse
     if request.user.is_authenticated:
-        return HttpResponseRedirect("/")
+        return HttpResponseRedirect(request.user.realm.uri)
     if is_subdomain_root_or_alias(request) and settings.ROOT_DOMAIN_LANDING_PAGE:
         redirect_url = reverse('zerver.views.registration.find_account')
         return HttpResponseRedirect(redirect_url)