Refs #969 - Proxy-side changes for serving templates from the proxy

Compliment to https://github.com/theforeman/foreman/pull/751

Probably also refs #1969 as `:template_url` is the client-facing proxy name, and so could be used for `puppetserver` too. `:foreman_url` could be used for implementing an auto-update-on-feature-change, if we want to.

There's an open issue about serving http on an https proxy, which will probably require a 2-threaded proxy listening on two ports. For now, this functionality is for http proxies only.

@domcleal, do we have somewhere to start putting documentation for 1.3 features?