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

An update to @GregSutcliffe's original PR (https://github.com/theforeman/smart-proxy/pull/100/files). Ports his original feature to the new plugin api.
There's a PR that makes it possible to reuse ForemanRequest (#223), which I expect is going to be merged shortly. Would be less duplication if you used that.