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

Needs the proxy-side PR to function.

This is a rebase of @GregSutcliffe's original https://github.com/theforeman/foreman/pull/751.
We have a classic duplication problem here:

Currently, the default action here is "built" while the default action `Foreman::Renderer` is "provision". You've nicely refactored this into a single method (good idea) but now, the default in `ForemanUrlRenderable` is "provision". That would cause all templates which rely on the default action on "built" (because of which foreman_url() implementation they hit) would break - meaning users would have to update all their templates, which I'd prefer to avoid.

@dustints - I think we should probably split this refactoring off into another Redmine/PR, if we can - I like this solution, but I'm not sure a refactor of Foreman::Renderer is suitable for a last-minute inclusion into 1.7. It certainly won't break this PR to instead update both the current foreman_url implementations. @domcleal @ohadlevy thoughts?