Disable Rails frameworks we know we won't use
Add Unicorn for running the app in production mode
Add logstasher for logging
Set assets prefix

We will have some of our own assets. This corresponds to:
https://github.gds/gds/alphagov-deployment/blob/9d0ee17fb94399e9b63ce7baebc0164a51911bc2/courts-frontend/config/deploy.rb#L13