This changes the email "From" field from "gitlab@example.com" to either:

* "John Doe <gitlab@example.com>" if the author of the action is known,
* "GitLab <gitlab@example.com>" otherwise.

Rationale: this allow mails to appear as if they were sent by the
author. It appears in the mailbox more like a real discussion between
the sender and the receiver ("John sent: we should refactor this") and
less like a robot notifying about something.