Refs #334 Improve delete contact button

In registration contrib app, on the edit contact page:
* Don't display delete button when adding a contact
* Make the "Save Contact" button green so it stands out
* Make the "Delete Contact" button gray so it doesn't stand out
* Make the "Delete Contact" button just popup a dropdown, in red, that
  says "Really Delete Contact" - user has to click a second time on the
  red button before the delete happens.
Merge pull request #334 from rapidsms/feature/334-contact-form-delete-button

Contact form: Delete and Save buttons are too similar