fix bug 1056950 - only send welcome email when at least one email address is verified

This also delays sending the welcome email till after the initial email address confirmation has happened.

To clarify why I emulate some parts of the signup process here: The Persona provider only knows verified email addresses which doesn't trigger the feature to be tested here. The GitHub provider on the other hand is hard to test due to Oauth2.
:+1: tests that explicitly create their own fixtures. Should we file a follow-up bug to create some User test factory methods, like [the ones in the wiki app](https://github.com/mozilla/kuma/blob/1954e547e91ecdb93ea79670aad2a35a2aa15803/kuma/wiki/tests/__init__.py#L38-L103)?