DEV: Always create chat mention records (#20470)

Before this commit, we created a chat mention record only in case we wanted to send a notification about that mention to the user. Notifications were the only use case for the chat_mention db table. Now we want to use that table for other features, so we have to always create a chat_mention record.

plugins/chat/spec/lib/chat_notifier_spec.rb

@@ -23,7 +23,7 @@ describe Chat::ChatNotifier do
     end
 
     def build_cooked_msg(message_body, user, chat_channel: channel)
-      ChatMessage.new(
+      ChatMessage.create(
         chat_channel: chat_channel,
         user: user,
         message: message_body,