mail_room was configured to deliver mail to the `incoming_email`
queue while `EmailReceiveWorker` was reading the `email_receiver`
queue. Adds a migration that repeats the work of a previous
migration to ensure all mails that wound up in the old
queue get processed.

Closes #23689

Added a comment to remember developers to open merge request to omnibus
in future changes of the file.

We will trust redis configuration params loading to Gitlab::RedisConfig.

Signed-off-by: Rémy Coutable <remy@rymai.me>

Improve and finish the fallback to the In-Reply-To and References header for the reply-by-email feature

A few things to note:
- The IncomingEmail feature is now enabled even without a
  correctly-formatted sub-address
- Message-ID for new thread mail are kept the same so that subsequent
  notifications to this thread are grouped in the thread by the email
  service that receives the notification
  (i.e. In-Reply-To of the answer == Message-ID of the first thread message)
- To maximize our chance to be able to retrieve the reply key, we look
  for it in the In-Reply-To header and the References header
- The pattern for the fallback reply message id is "reply-[key]@[gitlab_host]"
- Improve docs thanks to Axil

Fixes #12731