2225:, 2224: Fixed the way we generate billing reports for translations / test for completedness.

There were two major problems with the earlier code:
- We were checking language.is_complete instead of is_complete_and_synced which would generate billing records for languages that weren't synced (and therefore invalid records, invalid timming)
- If you created translations from unsyced source languages, once the source language was complete the billing records would not get created.

I've also moved the logic to teams.models, since it's not a task specific functionality. It should still be triggered by tasks, but it doesn't have to be there.
Merge branch 'prod-test' of github.com:pculture/unisubs into prod-test