Squash migrations

Debusine's test suite is excessively slow due to a large number of migrations that we probably don't need to keep around any more. Squashing all the migrations from before 0.11.0 cuts the test suite's runtime nearly in half, and makes Django-based tests much quicker to start up.

As well as the usual Django machinery to mark new migrations as replacing old ones, my plan is to include a preinst check in debusine-server that bails out before the upgrade if you're trying to upgrade directly from a version before 0.11.0, which should guard against accidents. In future, squashing once every Debian release cycle seems reasonable enough, although of course we can revise that if it turns out that we accumulate migrations even more quickly during the forky cycle than we did during the trixie cycle.