auth: Only automatically redirect for same domain redirects.

If the `deactivated_redirect` belongs to the same domain as
`EXTERNAL_HOST`, automatically redirect, otherwise just point
user to the new URL.

corporate/views/support.py

@@ -605,6 +605,7 @@ def support(
                 if parse_result.port:
                     hostname = f"{hostname}:{parse_result.port}"
                 subdomain = get_subdomain_from_hostname(hostname)
+                assert subdomain is not None
                 with suppress(Realm.DoesNotExist):
                     realms.add(get_realm(subdomain))
             except ValidationError: