Use alembic for database version migrations.

[ockham]

We might consider use of Alembic (which is said to be more lightweight than e.g. SQLAlchemy-Migrate) for migrating database versions which we're currently doing in one long function in backends/db.py.

Note though that normally, Alembic uses a hash revision tagging of its own which we'd need to override by our database version tagging (in the info table of our db). I've contacted the Alembic mailing list regarding this subject.