For T348388: Use central login wiki for login (SUL3), we want to move login to a central login wiki: when you click login / signup, you get redirected to (in the case of Wikimedia wikis) login.wikimedia.org, go through the user interactions there, and get redirected back at the end.

That means the login wiki needs to be able to simulate wiki-specific aspects of login/signup, most notably the AuthManager::canCreateAccount() checks as we want to apply local username policies, blocks etc. That will probably involve the CentralAuth authentication provider proxying the calls to ApiQueryUsers on the initial wiki (as otherwise running code in the context of a different wiki is a hard problem).