This changes the email subjects for issues and merge request
notifications from:

    Team / Project | Note for issue #1234

to:

    Team / Project | Saving issue doesn't work sometimes (#1234)

Rationale:

* Scan the subject of the email notification more easily when catching
up with a lot of notifications. Instead of having to open the email to
get the title of the issue or merge request, one can simply read the
subject of the email.

* Group messages by subject: email clients will group emails in threads
if they have the same subject.