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.
2271: show tasks link for team members that can create them
2275: fixed logic for disabling the edit subtitles button
2294: Don't unfork subtitle languages after submission

Changed it so that we only set is_forked to be True in
finished_subtitles, we never change it from True to False.  I'm pretty
sure there's no way that the editor should ever be able to unfork a
subtitle language.
2293: limit task deletion when a language is deleted

Before we would delete all tasks with that language code, now we only
delete tasks for the language's video.
2299: blind fix for filter.

I'm pretty sure what was happening was that team_video was None and we
were trying to filter on it.
2275: fixed logic for showing the edit button

Consolidated the code that checks if a user can add a new version into
1 place.  Previously it was in several places, so things could get out
of sync.  Like Janet's comment that we could show the edit
button, but not allow the edit to actually be performed.