Refactor to make it easier to extend.

* Put the logic entirely in the Auth_remoteuser class.
* Add separate, override-able methods for each stage of account creation.
* Override new methods where it may be necessary.

Change-Id: I16e2a0dfaf477a818aa555fe92f9b9497ade0e0d
Reviewed-on: https://gerrit.wikimedia.org/r/127406
Reviewed-by: Chad <chadh@wikimedia.org>
Tested-by: Chad <chadh@wikimedia.org>