Setup: Move wgActionPath logic to PathRouter

Setup: Move wgActionPath logic to PathRouter

This is only relevant when processing page views or when constructing
Title urls with an 'action' query. Pretty important stuff, and worth
optimising for if we had to choose, but we can defer it in this case
without slowing it down, which is better for everything else.

It also means we don't mutate configuration (beyond setting whole values
as dynamic defaults), which seems desirable, and makes the overall behaviour
easier to test. Handling absence of 'view' should be PathRouter's
responsibility, not Setup.

Bug: T189966
Change-Id: I9c1eea2dcea74be0e283eb2b175268315ced1793