Filter content by role

DON'T MERGE THIS YET! Still need to do work on [theodi/frontend-www](https://github.com/theodi/frontend-www) and [theodi/gds-api-adapters](https://github.com/theodi/gds-api-adapters)

I'm also not sure if we should be setting the role via HTTP header (which is what I'm doing at the mo), or GET request. HTTP header keeps the URLs nice, but is a bit obscure and may confuse people trying to use the content API.

Needed for theodi/shared#107