Require teamwork for all transcripts.

- Introduces TRANSCRIPTS_REQUIRE_TEAMWORK setting, default True. During testing, it's only set to True while testing the behavior of requiring teamwork. For tests the default is False for convenience in isolating the primary behaviors under test.

- Adds 'last_editor' field to TranscriptFragment and TranscriptStitch, and '[clean|boundary|speaker]_last_editor' fields to Sentence. These are updated during the _validate methods of each type of Task.

- Moves logic from transcripts.views to transcripts.models, resulting in existing_transcript_task() and assign_next_transcript_task()

- _available_fragments(), _available_sentences(), and _available_stitches() updated to take into account teamwork requirements and the last editor to work on a particular piece of a transcript. These are used by can_create() and create_next(), which in turn are used by assign_next_transcript_task().