FIX: Specific email error for replies to digest emails (PR #9770)

Emails the are replies to Digest emails, are getting rejected. Currently They are rejected by BadDestinationError. This is the catchall, that multiple cases result in.

This PR adds a check to make sure sure that the In-Reply-To header of incoming messages does not match the message_id of a Digest email that was sent out. If this is the case, we can send a much more specific error email, saying that you cannot reply to a digest email.

:tada: Email debugging is fun


I AM EXCITE :tada:

        raise ReplyToDigestError if EmailLog.find_by(email_type: "digest", message_id: @mail.in_reply_to).exists?

@eviltrout I commited this and got undefined method "exists?" for EmailLog. Switching back to .present?`

Ah NVM, I switched the find_by to where so that .exists? should work.

Oh my bad it should be EmailLog.where(email_type: "digest", message_id: @mail.in_reply_to).exists?