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/db/post_migrate/20230227172543_make_chat_mention_notification_id_nullable.rb

@@ -0,0 +1,11 @@
+# frozen_string_literal: true
+
+class MakeChatMentionNotificationIdNullable < ActiveRecord::Migration[7.0]
+  def up
+    change_column_null :chat_mentions, :notification_id, true
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration
+  end
+end