CI: Make a useful migration test

Created by: efritz

This PR was originally meant to make the CI check for backwards/forwards runnable migrations more useful (by removing a piece of easy code that prevented us from seeing a bunch of problems).

This revealed a few such problems! I've updated the migrations that can't be ran twice so that the test will initially pass. I've also discovered an off-by-one error in the downgrade path for the new migration code, fixed here.