somehow not adding the returnurl param went missing

[rschrieken]

this fixes https://meta.stackexchange.com/questions/256042/sede-login-redirects-to-front-page-rather-than-returning

I've simply copied the build-up of the returnUrl query parameter as it was done in Views\Account\LogInActiveDirectory.cshtml

Tested with the SE OpenId provider. Any other provider that supports keeping the returnurl intact would work similar.

[NickCraver]

Note for self: we need to ensure there's not an open redirect here on the server implementation (relative or current domain should be allowed only) - want to investigate this before merging in another path to it.