notify: reply to notification mail to comment #295

Notification mail is outbound only. Replying to it should post a comment on the thread.

• Each notification carries Reply-To: reply+<token>@<domain>; the token binds recipient, repository and issue/MR, and expires.
• Inbound: either an SMTP listener in gitbayd behind an MX record, or polling a mailbox over IMAP. The second needs no open port.
• The reply is checked against the recipient's current rights; a revoked or disabled account posts nothing.
• Quoted text and signatures are stripped; the body is stored as markdown.
• Refusals send nothing back (no backscatter), recorded in the audit log.
• Setting per user, off by default, next to notifications settings mail.