DEV: Create a chat_mention record when self mentioning (#21438)

In the past, we create a `chat_mention` records only when we wanted to notify a user about a mention. Since we don't send notifications when a user mentioning himself, we didn't create a `chat_mention` records in those cases.

Now we use `chat_mentions` records in other scenarios too, so when a user is mentioning himself we want to:
1. Create a `chat_mention` record for that mention
2. Do not create a notification for that mention

plugins/chat/lib/chat/notifier.rb

@@ -132,6 +132,7 @@ module Chat
           .global_mentions
           .not_suspended
           .where(user_options: { ignore_channel_wide_mention: [false, nil] })
+          .where.not(username_lower: @user.username_lower)
           .where.not(id: already_covered_ids)
           .pluck(:id)
 
@@ -150,6 +151,7 @@ module Chat
           .here_mentions
           .not_suspended
           .where(user_options: { ignore_channel_wide_mention: [false, nil] })
+          .where.not(username_lower: @user.username_lower)
           .where.not(id: already_covered_ids)
           .pluck(:id)
 
@@ -187,9 +189,12 @@ module Chat
         direct_mentions = []
       else
         direct_mentions =
-          @chat_message.parsed_mentions.direct_mentions.not_suspended.where.not(
-            id: already_covered_ids,
-          )
+          @chat_message
+            .parsed_mentions
+            .direct_mentions
+            .not_suspended
+            .where.not(username_lower: @user.username_lower)
+            .where.not(id: already_covered_ids)
       end
 
       grouped = group_users_to_notify(direct_mentions)
@@ -209,6 +214,7 @@ module Chat
           .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)
 
       too_many_members, mentionable =

plugins/chat/lib/chat/parsed_mentions.rb

@@ -91,7 +91,6 @@ module Chat
         .joins(:user_option)
         .real
         .where(user_options: { chat_enabled: true })
-        .where.not(username_lower: @message.user.username.downcase)
     end
 
     def parse_mentions(message)