DEV: Simplify discourse_dev postgres setup (#830)

DEV: Simplify discourse_dev postgres setup (#830)

- Remove manual database creation, and instead promote discourse user to postgres SUPERUSER. This means that `db:drop` and `db:create` commands can be run in the dev image, just like in other local development environments. As well as simplifying things, it fixes turbo_rspec, which was previously impossible in the docker dev environment (because `discourse` didn't have permissions to create the parallel databases)

- Stop pre-migrating test database in dev image. It adds additional build time & image size, and doesn't actually help because core's `bin/docker/boot_dev` script overwrites the container's postgres directory with a volume mount