Disable app resource in convox.yml

We use app-level database resources for several staging environments, but our production environment uses a rack-level resource. Currently to support this, we alter our convox.yml file for the production environment, removing the app resource definition.

It would be nice if this could be handled in convox.yml, for example:

resources:
  database:
    type: postgres
    enabled: ${PG_ENABLED}

If there’s some interest, I might be able to send a PR for this.