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.