This finishes off the base support for database evolution using Django

2.1, building on some of the changes recently committed. This primarily

entailed calling some functions with some proper arguments, ensuring

pickled data can be saved in models, temporarily disabling

constraint checking when evolving, and updating tests to check for 2.1

SQL output.
Django 2.0 itself is being skipped, due to index creation bugs for

SQLite that break tests, due to index names failing to be escaped.
Like with anything post-Django 1.6, there's still a lot of work that

needs to be done in order to really integrate this with a project, which

will happen before the final release. migrate and evolve still don't

play nice. When that's addressed, database evolutions and migrations

will co-exist for all supported versions of Django.

Unit tests pass on all supported versions of Django, across all supported

database types.