[WIP] Translate contact details

Outstanding tasks

- [ ] Add contact numbers to translation form
- [ ] Add test for display of translation on organisation pages (the functionality Just Works)
- [ ] Make worldwide offices translatable

There are now 8 admin controllers for handling translations, and I expect to create a 9th to handle worldwide offices. They are all doing more or less the same thing in more or less the same way, so seem like 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`.