Database migrations

Me and @johan-bjareholt have had some discussion about this since it's something we'd need if we want to change the database schema between versions once we have real users.

I looked around for migration tooling and found the minimalistic caribou: https://github.com/clutchski/caribou

Last commit was 7 years ago and only seems to support Python 2.6 but the code base is really small (271 LOC) and I wouldn't mind forking it and updating it myself. Unless there is something equally simple out there.