While working on T362713: Implement the new login process which redirects to the central login wiki for showing the login/signup form, it was noticed that LoginSignupSpecialPage::getReturnToQueryStringFragment() is used in some places where ::getPreservedParams() should be used instead.

Code in the LoginSignupSpecialPage class and possibly other places should be refactored to use getReturnToQueryStringFragment() and/or getPreservedParams() in the correct places so that extensions such as CentralAuth can hook into that code or code calling those methods to preserve the query params when authentication process is happening.

The goal of this is for extensions to be able to make use of query params (GET params) during the authentication workflow between wikis.