1894: Fix crediting where last sub is unsynced

* Often, the last subtitle won't be synced and will just extend until the end
  of the video.  In those cases, the "end" value is -1.  If that's the case, we
  can't add the credit sub.

* Made the add_credit() function dumber and easier to test (it now takes the
  list of subs, the language code and the video duration).  This makes it much
  easier to test.

* Rewrite tests (see above)
2025: Sync to YT when marking lang complete in admin
2107: Disable 'perform task' when task is locked.
2122: fixed duplicate wistia entries causing media compilation to fail
Merge branch 'production' into i-2025
2025: Only sync when language becomes complete
2107: Hide everything but assignment info when task is blocked
Merge branch 'i-2107' of github.com:pculture/unisubs into i-2107
Merge branch 'production' into i-2107
Merge branch 'i-2124' into production
Merge pull request #129 from pculture/i-2107

2107: Fix blocked tasks styling
Merge branch 'i-2025' into production
Merge branch 'i-2124' into production
Merge branch 'production' into i-1894

I'm pretty sure something will break as a result of this... Just come yell at
me.

Conflicts:
	apps/videos/tests.py
	apps/videos/types/youtube.py
Merge branch 'i-1894' into production
Merge branch 'i-2124' into production
2093: Ensure empty descs are handled properly
Merge branch 'i-2093' into production
2151: Add sorting on "created"

Missing db field, "created" on TeamMember.  Can be inferred from the Activity
model.
2151: Add migrations