message: Optimize checking if mentioning user group is allowed.

This commit updates the code to not call is_user_in_group
function if can_mention_group setting is set to "role:everyone"
group.

zerver/lib/message.py

@@ -1220,12 +1220,12 @@ def check_user_group_mention_allowed(sender: UserProfile, user_group_ids: List[i
 
     for group in user_groups:
         can_mention_group = group.can_mention_group
-        if sender_is_system_bot:
         if (
             hasattr(can_mention_group, "named_user_group")
             and can_mention_group.named_user_group.name == SystemGroups.EVERYONE
         ):
             continue
+        if sender_is_system_bot:
             raise JsonableError(
                 _("You are not allowed to mention user group '{user_group_name}'.").format(
                     user_group_name=group.name