Squash django migrations and remove django-fsm references (#4336)

* Django FSM migration to Viewflow

- Added new `django-viewflow` and `django-filter` dependencies to `requirements.txt`.
- New file `viewflow.py` under `/audit/models/` which contains the FSM logic for transitioning an SAC.
- Moved `STATUS` enumeration outside of the `SingleAuditChecklist` class. This required import/reference changes across many files and tests.
- Removed references of old deprecated library `django-fsm`.
- New migration to handle the changing the SAC's `submission_name` field to remove dependency on the deprecated `django-fsm`.

* Git conflicts with #4292

* Squash migrations

* Remove django-fsm requirement

* Update requirements.txt

* Re-add django-fsm dependency

This cannot be done until the migration squash is deployed to production.