fix redirect after login

[joschahenningsen]

Motivation and Context

The redirect after logins is currently broken. This fixes #1053

Description

The redirect target cookie is now properly parsed and set when visiting the login page as opposed to when submitting the login form. This fixes issues with saml logins.

[github-actions]

Your Testserver will be ready at https://1056.test.live.mm.rbg.tum.de in a few minutes.

Logins

Kurs1	Kurs2	Kurs3	Kurs4
public	public	loggedin	enrolled
prof1	prof1	prof2	prof1 prof2
student1 student2 student3	student1 student2	student2 student3	student1 student2

[joschahenningsen]

I removed the frontend logic of using the document.referrer for this case, as it only worked when the browser set it and when using the login form. Now, the referrer cookie is set upon loading the login page when the login page has the ?return=... query. (Which is the case when a page was accessed that requires login but no session is present)

[alexanderstephan]

I can't really test this right now, but the changes look good to me. I think the cookie workaround is perfectly sane.