fixes #996 escape word "groups" in SQL queries, new reserved word with MySQL 8

admin/user_list_backend.php

@@ -203,7 +203,7 @@ SELECT
     user_id,
     GROUP_CONCAT(name ORDER BY name SEPARATOR ", ") AS groups
   FROM '.USER_GROUP_TABLE.'
-    JOIN '.GROUPS_TABLE.' ON id = group_id
+    JOIN `'.GROUPS_TABLE.'` ON id = group_id
   WHERE user_id IN ('.implode(',', $user_ids).')
   GROUP BY user_id
 ;';