Squash migrations

This is basically the same I already proposed in !1503 (closed), but this time I think we should merge it.

The MR in its current form only adds squashed migrations, basically shortcuts for creating new databases.

A commit to remove the old migrations is included, but reverted. We can still decide if we want to cherry-pick it later.

If we decide to do that we would already reduce the number of migrations considerably. Some of the more complicated migrations would still exist (!1614 (merged), !1581 (merged)), but I believe squashing them will be easier after this.