Add option to disable searching in the user dir

We still populate it, as it can still be accessed via the admin API.

synapse/rest/client/v2_alpha/user_directory.py

@@ -59,6 +59,12 @@ class UserDirectorySearchRestServlet(RestServlet):
         requester = yield self.auth.get_user_by_req(request, allow_guest=False)
         user_id = requester.user.to_string()
 
+        if not self.hs.config.user_directory_search_enabled:
+            defer.returnValue((200, {
+                "limited": False,
+                "results": [],
+            }))
+
         body = parse_json_object_from_request(request)
 
         limit = body.get("limit", 10)