FIX: Further optimize mentioning groups in chat messages (part 2) (#24185)

This is a follow-up to e6299a3. I additionally fixed these three things:

1. Since e6299a3 there's no need anymore to join the group_users table 
when looking for users who were reached by a group mention, so 
I removed that join in that commit. But turned out we were joining 
the group_users table twice, so I removed the second join in this PR. 
That drastically speeded up my test query, from 6 sec to 0.26 sec.
2. We also were joining twice the user_chat_channel_memebership table, 
so I removed the second unnecessary join too.
3. We actually need to join the user_chat_channel_memebership table 
only in certain cases, and we don't need to do that for group mentions, 
so I fixed that too.

As a result of these changes, time of my test query fall down from 
6 sec to 0.001 sec. And the resulting SQL query now contains only 
one JOIN statement.

plugins/chat/lib/chat/parsed_mentions.rb

@@ -89,7 +89,7 @@ module Chat
     private
 
     def channel_members
-      chat_users.where(
+      chat_users.includes(:user_chat_channel_memberships).where(
         user_chat_channel_memberships: {
           following: true,
           chat_channel_id: @message.chat_channel.id,
@@ -98,13 +98,7 @@ module Chat
     end
 
     def chat_users
-      User
+      User.distinct.joins(:user_option).real.where(user_options: { chat_enabled: true })
-        .includes(:user_chat_channel_memberships, :group_users)
-        .distinct
-        .joins("LEFT OUTER JOIN user_chat_channel_memberships uccm ON uccm.user_id = users.id")
-        .joins(:user_option)
-        .real
-        .where(user_options: { chat_enabled: true })
     end
 
     def mentionable_groups