* new merge request can be created by sending an email to the specific
email address (similar to creating issues by email)
* for the first iteration, source branch must be specified in the mail
subject, other merge request parameters can not be set yet
* user should enable "Receive notifications about your own activity" in
user settings to receive a notification about created merge request
Part of #32878
- allows unsubscription processing of email in format "reply+%{key}+unsubscribe@acme.com" (example)
- if config.address includes %{key} and replies are enabled every unsubscriable message will include mailto: link in its List-Unsubscribe header