Use alembic for database table creation

[jstutters]

Allows us to be able to control the upgrade of the pixl database tables in production with upgrades and rollbacks

[stefpiatek]

Really nice, thanks for your speedy work on this. I think we want to have this run when the imaging-api starts up because the postgres init db script only runs if there is no database. That way we can update the schema without losing any data

[stefpiatek]

Code changes look perfect, assuming it runs! I think there's also some documentation we could add to the alembic README. Happy for you to merge once you're happy with it

Picked this up so my review shouldn't count

[milanmlft]

Not really familiar with Alembic, so probably not the best judge but it looks good to me.
A bit more documentation would be nice though 😬

[peshence]

Looks good, huge improvement once it's over the line. Couple of small comments/questions here and there.

Also, shouldn't this be part of the core module?