Replaced the unpublish button

We decided to ditch the unpublish button and all the complexities that
go with it.  Instead, we now have a button to delete an entire
subtitle language

This deletes all SubtitleVersions in that language, as well as any
dependent languages.  This is the nuclear option for unpublishing
subs, and should only be used in extreme cases (like a language whose
only versions are spam).
2270: 2268: Delete tasks assocated with a language when it's deleted.

There were a couple ways to implement this one, I choose this method
because it avoiding adding a circular import between the subtitles app
and the teams app.
2275: Edit Subtitles button.

If a version is made private by a superuser, then the button should be
enabled.  However, we should still keep the old code that disables it
for in-progress tasks.
2276: Don't block tasks for languages with a non-public last version

This is possible if a superuser makes the last public version private.
The review tasks should still be enabled, they should just use the
latest version that is public.
2269: delete translate tasks when we delete a language

Only if there arent other public subtitles that can be used for the
translations.