set up CI

since donate-neo isn't using lektor and isn't a static site, we can't re-use any part of the donate-static CI config. which means we get to make our own (yay!)

the minimum for CI would be running flake8 (linting), mypy (in permissive mode, django doesn't typecheck well), and running tests and reporting test coverage. as of now, most of those tests would fail

but it would also be nice to have a staging environment and previews for merge requests. that would require some work on the TPA side to set up django review apps