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

_Originally opened at !3024 by @dabit._

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Fixes #2364 

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

See merge request !3305