Fall back to In-Reply-To and References headers when sub-addressing is not available

Originally opened at !3024 (closed) by @dabit.

---

Fixes #2364 (closed)

Summary of the changes:

• No more need to have the %{key} placeholder in the incoming_email.address
• The fallback message id format is reply-[key]@[gitlab_host] (reminder: it doesn't have to be a real email address)
• The fallback message id that includes the reply key is added to both References header
• Documentation for the "Reply by email" feature updated