Get publishing-api working

In order to support publishing to both a draft and live environment we need an app to sit in front of the content-store(s) and route items accordingly.

This is the first step towards that. It passes content to the live content-store. For the time being it only allows configuring a single content-store.

Story: https://www.agileplannerapp.com/boards/173808/cards/8693
On 4xx responses from the URL arbiter, we want to be returning the same status along with error details in the same way that [the content-store currently does](https://github.com/alphagov/content-store/blob/master/app/controllers/content_items_controller.rb#L46-L66).