bug 1113252 - send edited notification email for users' first edits

To spot-check:

* [ ] Create a new user
* [ ] Make a first edit (or a new page) with the new user
* [ ] Should see [the edited notification email](https://github.com/mozilla/kuma/blob/master/kuma/wiki/templates/wiki/email/edited.ltxt) in the console output
* [ ] Make another edit with the new user
* [ ] Should see no email in the console output
I wanted to do `send_first_edit_email.delay(new_rev, request)` but I ran into [the issue where I couldn't pickle the request object](http://stackoverflow.com/questions/3103638/pickling-django-request-objects). Not sure how to avoid it, without [introducing a whole bunch of new code](http://stackoverflow.com/questions/2349483/django-models-signals-and-email-sending-delay)?