Set up database migrations

[bendnorman]

Currently, to make changes to data that already exists in the database you need to clobber the whole table. This is not ideal because the dashboard might experience downtime.

Instead, we should use a database migration tool like [alembic](https://alembic.sqlalchemy.org/en/latest/) so we can apply changes to the database without having to clobber data.

Resources

• alembic docs
• Migrating a db without downtime.