Squash alembic migrations; sync with data models; add SQL cascades

Description

This MR squashes all earlier alembic migrations into one; This makes it so that new users only need one migration to create the current, more mature version of the database.

Our data model definition was a bit out of sync with the alembic migrations, and this MR also syncs those up. In addition to that, this MR also adds SQL on delete/update cascades, so that jobs can be easily deleted with their relevant leases and operations with a single query.

Relevant mailing list post: https://lists.buildgrid.build/pipermail/buildgrid/2020-February/000170.html