Validate user id for users select autcomplete

Single user autcomplete should be used only for existing users with
digital ID provided. Now js code puts any input into generating user URL
which can lead to 500 error because routing like this does not exists:

GET "/autocomplete/users/whatever@example.com.json".
Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

app/assets/javascripts/users_select.js

@@ -325,6 +325,10 @@
     };
 
     UsersSelect.prototype.user = function(user_id, callback) {
+      if(!/^\d+$/.test(user_id)) {
+        return false;
+      }
+
       var url;
       url = this.buildUrl(this.userPath);
       url = url.replace(':id', user_id);