Improve new-style controller selection by relying on ContentNegotiation

The goal is that new-style SCOTCHish controllers should be used to serve all text/html requests that do not have an explicit controller set. This patch updates the RouteProcessorSubscriber to actually *do* that - right now, it selects the new style controller if the current route has no controller (correct), and has a _content attribute set (irrelevant).

of course, we don't have the new style controller yet, just the basic passthrough one that Larry wrote. nevertheless, this gets us a step closer.