Push event webhook should include username of pusher
Description
The push event webhook doesn't include the username of the user who did the push. This would be useful for authorization -- maybe I only want certain users to be able to push to certain branches or files. There is the "user_id" field, which would let me get the username from the user API, but that's a hassle. There's also a field called "user_name", which is actually the user's display name. That is, I want (from the db's perspective) the username field of the user table in addition to the name field. My assumption is that in a Kerberos world this corresponds to the Kerberos principal.
See also #2195 (closed) by @abliss.
Proposal
Add a user_username field. Looks like about a three-line patch. I'll try to submit it later today if it turns out to in fact be as straightforward as it looks.