When a merge request is created from email, use message body
as merge request description. If message body is empty then
merge request description is still created from the source
branch commit (if there is only single commit in the merge
request).
If message body is empty and there are multiple commits in
the source branch, then merge request description is left empty.
Closes#40968
* 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
