Implemented 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).  It is only available to staff users.
Improvements for unpublishing private subtitle versions

Now if a SubtitleVersion is private (because it's not been approved,
or because it already has been unpublished), we show a button/form
that says Delete instead of Unpublish with an option to delete.