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)
1890: Fix unittest logic
1980 Sync teamvideos linked to individual accounts
Merge branch 'production' into i-1890

Conflicts:
	apps/accountlinker/models.py
	apps/accountlinker/tests.py
1890: Fix check_authorization logic
1890: Fix unittests (yt imported versions)
1890: Fix more tests
2025: Sync to YT when marking lang complete in admin
Merge branch 'production' into i-1890
Merge branch 'production' into i-1890
1890: Sync to youtube when teamvideo is deleted
2107: Disable 'perform task' when task is locked.
2124: Add BillingRecord model
2124: Add post_save hook to create BillingRecord
2124: Add billing cut off date
2122: fixed duplicate wistia entries causing media compilation to fail
2124: Add BillingRecord admin
2124: Move BillingRecord insertion to task
2124: Beef up BillingRecord admin
2124: Add billing cutoff date to billing view