Allow /content/zipcontent endpoint to accept a baseurl GET parameter for remotely accessing zipcontent

[rtibbles]

An API endpoint setup under /content/zipcontent that will proxy to a specific remote server using the get param baseurl specified. If no baseurl is specified, it should return a 404.

The API endpoint will then make a further request to /content/zipcontent/.... and then stream the response from that request as the response to the request being made from the client.

Caching these resources is a little trickier, unless we took the decision to stream the entire zipcontent file and attempt to serve the embedded resources from that.