Migrate from django-fsm to viewflow

[danswick]

As of April this year, django-fsm, the state machine library we use to to manage submission events and the like, has been deprecated. Its replacement, viewflow, is a backwards-incompatible re-write of the old system. While it seems to be a straightforward replacement, it will require some work to migrate.

Update:
As @rnovak338 pointed out, we import django-fsm in some old migrations, which means we can't easily remove the dependency without also doing some minor migration surgery. With that in mind, we'll probably do something like:

Steps

Beta Give feedback

1. Refactor the app code to use the viewflow API, add a new migration that changes the FSMfield to a standard CharField
2. Squash the existing migrations to get rid of the new-deprecated fsm reference. Or possibly some other approach to fixing the migrations.