LoginSignupSpecialPage: Load return and returnto params as early as possible

LoginSignupSpecialPage: Load return and returnto params as early as possible

They're are needed for a redirect to the target page after
a successful login, which is made before the SpecialPage::execute()
function is called. Loading basic request varaibles in the execute()
function is therefore too late to take effect for the redirect after
a successul authentication with a primary provider, which needs to
redirect the user to another site.

Bug: T135924
Change-Id: I6ded7f9bb255cbb332a5810e7ed3cb3ecfdb2c04