For SUL3 (T348388: SUL3: Use a dedicated domain for login and account creation) we want to ensure that access to the central session cookies is preceded by user interaction. When the user is not centrally logged in, that means we render the log in form on the central domain (which counts as a user interaction, as required by browsers to reliably store cookies on the central domains).

When the user is already centrally logged in, we currently redirect straight back to the local wiki and log the user in there. This is great for user experience, but browsers might penalize the cookie retention if there is no interaction there like there is on log-in. This task is to determine whether it is realible or not.

Otherwise, we might need to create an interestitial there with some trivial user interaction, such as clicking a button. (Other websites usually show a mini profile with some sort of "You are logged in as XX, continue / switch account" choice.)

Implementation
This can't be put in the authentication provider since the authentication provider is not invoked when the user is already logged in. We'll probably have to create a special page for it, and make the redirect flow either home wiki -> loginwiki:Special:LoginInterstitial?returnurl=... with the special page requiring login (and thus naturally redirecting to the login page), or home wiki -> loginwiki:Special:Userlogin?returnto=Special:LoginInterstitial&returnurl=....

Not sure what to do about API login/signup. I guess we can just ignore that because we couldn't guarantee user interaction there anyway.