Added $wgCentralAuthLoginWiki option.

* This changes login to redirect to a central login wiki and back to set
  the cross-domain cookie for that wiki. This works around browsers not
  letting third party domain cookies get set directly.

Requires change Idd71cac47195402ebffeb7047e91efb200a32b58 to core.

Bug: 46903
Change-Id: Ie3d6523d29ed294c9d9ad44494d2e8f78ff330e9