FIX: Further optimize mentioning groups in chat messages (#24122)

A follow-up to faac6773. This PR eliminates one more heavy join by forcing 
Active Record to do two queries instead.

Also, along the way, I made this change:
```
# this generates two quries to the groups table
def groups_to_mention
  @groups_to_mention = mentionable_groups - groups_with_too_many_members
end

# so I changed it to (this makes only one query to the groups table):
def groups_to_mention
  @groups_to_mention ||= mentionable_groups.where("user_count <= ?", SiteSetting.max_users_notified_per_group_mention)
end
```
This one is kind of a premature optimization, because we don't have evidence that 
this extra query is a problem, but it seems cleaner this way.

Commits history on this PR may help better understand the change.

plugins/chat/lib/chat/notifier.rb

@@ -198,7 +198,6 @@ module Chat
         @parsed_mentions
           .group_mentions
           .not_suspended
-          .where("user_count <= ?", SiteSetting.max_users_notified_per_group_mention)
           .where.not(username_lower: @user.username_lower)
           .where.not(id: already_covered_ids)
 

plugins/chat/lib/chat/parsed_mentions.rb

@@ -51,8 +51,9 @@ module Chat
     end
 
     def group_mentions
-      mentionable_groups_ids = mentionable_groups.pluck(:id)
-      chat_users.includes(:groups).joins(:groups).where("groups.id IN (?)", mentionable_groups_ids)
+      group_ids = groups_to_mention.pluck(:id)
+      group_user_ids = GroupUser.where(group_id: group_ids).pluck(:user_id)
+      chat_users.where(id: group_user_ids)
     end
 
     def here_mentions
@@ -64,7 +65,11 @@ module Chat
     end
 
     def groups_to_mention
-      @groups_to_mention = mentionable_groups - groups_with_too_many_members
+      @groups_to_mention ||=
+        mentionable_groups.where(
+          "user_count <= ?",
+          SiteSetting.max_users_notified_per_group_mention,
+        )
     end
 
     def groups_with_disabled_mentions