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.
could you add a comment here reminding users to ensure the :foreman_url setting is set in the config file? I forgot to do that, and it caused me no end of trouble :)