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
web-platform-tests/wpt#3867.
* Make matching for special names ASCII case-insensitive. Fixes #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: #2126.

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.