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, such as i18n customizations (which are used e.g. to link to username policies) and the SpecialCreateAccountBenefits hook. We'll probably need to fetch the relevant data via a server-to-server API call, and override the relevant parts of the special page logic.