Revamp the rules for choosing a browsing context

This makes several changes:

* Stop throwing an exception in `<a>` and `<area>` activation behavior as a
  result of popup blocking as it doesn’t match implementations. Fixes whatwg#2616. 
  Formal testing is not possible and tracked by
  web-platform-tests/wpt#3867.
* Make matching for special names ASCII case-insensitive. Fixes whatwg#2443.
* Centralize all user-configurable behavior instead of having it
  duplicated in various ways in all the caller algorithms.
* Call out a known issue with browsing context name matching: whatwg#1440.

It also modernizes the writing style and makes variables and what is
returned much more explicit, including no longer relying on some
out-of-band channel for communicating whether a new browsing context
got created.