Replace fixtures file with one that uses /en-gb/ prefix
Improve sandbox gateway page

- Use i18n URLs
- Add breadcrumbs
Remove Rosetta
Improve forgotten password process

- Use correctly marked-up headings
- Ensure only active users can reset password
- Improve message on "done" template as Django 1.6 now allows form to
  validate even if there is no user with the given email address (so as
  not to leak information).