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

https://bugzilla.mozilla.org/show_bug.cgi?id=1113252

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
The address is mdn-spam-watch@mozilla.com.