Translate contact details

Story: https://www.pivotaltracker.com/story/show/53558977

Outstanding tasks

- [x] Add contact numbers to translation form
- [x] Correctly display RTL languages in admin form
- [x] Make worldwide offices translatable

There are now 9 admin controllers for handling translations. They are all doing more or less the same thing in more or less the same way, so might be a good candidate for drying up.

We should also remove the obsolete columns from `Contact` and `ContactNumber` once this has been released since they're all migrated to `Contact::Translations` and `ContactNumber::Translations`.