Move to Postgres

[finnbar]

SQLite is fine, but not ideal for the concurrent accesses which tablebot relies on. We should move to Postgres.

• Work out what CompSoc provides in terms of database access.
• Spin up a Docker image for development (e.g. https://github.com/mbg/network-wait/blob/main/docker-compose.postgresql.yaml)
• Replace any necessary Persistent calls.
• Work out how to migrate SQLite -> Postgres.
• Modify backup script to use Postgres.

[L0neGamer]

Compsoc should be able to provide credentials for postgres, and possibly mysql although that's less clear.