WIP: moving to alembic

[jdavcs]

Ref #10369

Very much WIP. First steps towards moving us to alembic. The new tests in the PR pass, the rest breaks, for now.

The alembic stuff is in model/migrations. There's a run function that handles the basic logic + a DBManager class that takes care of all db/alembic interactions. The steps are as follows:

1. no database >> create, initialize, add alembic
2. empty database >> initialize, add alembic
3. nonempty database, alembic, up-to-date >> do nothing
4. nonempty database, no migrate, no alembic >> fail, error message: require manual upgrade
5. same + automigrate >> same: fail, error message: require manual upgrade
6. nonempty database, has migrate, no alembic >> fail, error message: run db_manage script
7. same + automigrate >> (we use SQLAlchemy migrate to upgrade to alembic revision#0 (when alembic was added), add alembic, upgrade to current revision)
8. nonempty database, has alembic >> fail, error message: run db_manage script
9. same + automigrate >> (upgrade to current revision)

My idea is to provide an automated upgrade path for db versions that already have SQLAlchemy Migrate, but not prior to that (I assume there's no reason to go that far back). This will be simpler than the current logic. I think we can phase out this path in 1-2 releases - that way we'll gradually remove SQLAlchemy Migrate from the code base.

The current implementation is far from complete. Also, I did not include views and triggers intentionally - I think it will make more sense to let alembic handle them (as a revision). Or maybe not.

[jdavcs]

superseded by #12869