Move from ActionController::Renderer to ActionView::PartialRenderer

[julianrubisch]

Right now futurism relies on a ActionController::Renderer with a complex logic to resolve the correct controller for a given URL etc.

Turns out this might not be necessary, we could just use ActionView::PartialRenderer if we're able to obtain the correct view_context, which would drastically simplify things (and perhaps also speed them up a bit)