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
I'm not sure I understand what this block is for.  It seems to create a `bytes.Buffer` from the byte slice, only to read it out into another byte slice.