R1:9352c76e818a

Decouple some aspects of request routing and construction

Summary:
Ref T5702. This is a forward-looking change which provides some very broad API improvements but does not implement them. In particular:

- Controllers no longer require `$request` to construct. This is mostly for T5702, directly, but simplifies things in general. Instead, we call `setRequest()` before using a controller. Only a small number of sites activate controllers, so this is less code overall, and more consistent with most…