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.
Remove SQLite from the Gemfile and all references to a database

- If we have a database, it won't be SQLite.
- Remove Spring as it doesn't offer us anything.
- Pin the rails-api version.
Don't specify a Ruby version in bin/* files
Remove .keep files for assets, models and concerns
Remove ActionMailer and ActiveRecord
We don't need locale files or route examples
Keep public/ for potential future use, but remove its contents
Merge pull request #2 from alphagov/add_initial_rails-api_app

Add a skeleton of a Rails API app