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
