Add a skeleton of a Rails API app

- Generated with `rails-api new courts-api --skip-sprockets --skip-javascript --skip-test-unit --skip-spring` as per the HMRC Manuals API.
- Including Airbrake and Logstasher for us to be able to use the app in preview will be done in a separate PR.
We should disable some of these (action_mailer and active_record for sure), but it doesn't have to be done right now..