Fix Bug 1049964 - Buttons on Email addresses page

Supersedes https://github.com/mozilla/kuma/pull/2728 with new user fixtures and a new test. The original [spot-check instructions](https://github.com/mozilla/kuma/pull/2728#issue-42097490) still apply.
For the same reason we format beautiful notification boxes instead of alerts -- they're inelegant and we can do better.  I'd prefer no confirmation to this.