Cleanup email form when pending change

* Add missing Bootstrap 3 classes
* Move cancel and resend buttons beneath form field
* Visually separate the change email form and the resend email form
Improve re-sent email copy

“Successfully resent email change email” is a bit of a mouthful. Also,
resent is not re-sent.
Merge pull request #275 from alphagov/pending-email-change-form

Cleanup change email form when there's a pending change