Switch to using rails-api as the base for this.

This is Rails with the bits that aren't necessart for api-only apps
removed.  See https://github.com/rails-api/rails-api#readme for more
details.

This is the result of merging the output of `rails-api new content-store
--skip-active-record --skip-sprockets --skip-javascript
--skip-test-unit`
with all the changes we'd made to the app so far.
Remove the secret_token setting.

Leaving this file in place so that if (when?) this is upgraded to Rails4
we can find this comment, and add the necessary setting.
Merge pull request #5 from alphagov/rails-api

Switch to using rails-api as the base for this.