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.
@stephaniehobson all (i think) jqueryUI components have decent a11y nowadays. have a look at http://hanshillen.github.io/jqtest/#goto_dialog for example :grinning: