Allow admin user to remove translation for world location.
Allow admin user to remove translation for worldwide office.
Extract remove translation duplication into a Translatable module.

I did consider adding the `remove_translations_for` method to
`LocalisedModel`, but the method is fairly tightly coupled to
`ActiveRecord` and `LocalisedModel` currently isn't. Anyway, this
`Translatable` module may not be its final resting place, but it'll do
for now.
Process govspeak in admin edition translation preview.

This appears to have been an oversight in a previous story [1].

[1] https://www.pivotaltracker.com/story/show/43569731
Simplify assertions for admin edition translation preview.

By using a locale-specific CSS class, we can simplify the existing
assertions and make it easier to add some new ones in a later commit.
Convert the edit edition translation link into a button.

I'm planning on adding a delete button to go along with the edit button
and by moving the button "into" the relevant translation section, we
gain extra context for the button and avoid the need to say what it will
allow you to edit.

The markup/styling may not be great, but I think it gets me a bit
further forward.
Allow admin user to remove translation for edition.

The delete button is shown within the section for each translation to
make it clear to which translation it refers.

The delete button is only displayed if the edition itself is deletable.
This means that it will not be possible to delete the translation for a
published edition i.e. similar to the workflow for supporting pages.

I've chosen to add the delete button to the translations tab of the
admin editions show page rather than introducing an equivalent of the
supporting page show page for each translation. Doing it this way is
consistent with the existing edit button and reduces the number of steps
the user has to perform in order to delete a translation.

I've chosen to include the `::Translatable` module into
`Edition::Translatable`. It's possible that some of the methods on the
latter might be moved onto `::Translatable`, i.e. the ones that are
independent of editions.
Add notices to give feedback on admin translation operations.

I decided not to add specific tests for these flash notices, because
they are somewhat incidental and I feel that there is already sufficient
test coverage to catch any silly mistakes.
Merge pull request #270 from alphagov/admin-remove-translations

Allow admin users to remove translations