Redirect bruger efter login

[rasmusselsmark]

Se #762

Brug next query string parameter til at redirect'e brugeren tilbage til f.eks. aktivitetsside efter login. Brugen af next er dokumenteret på f.eks. https://docs.djangoproject.com/en/4.2/topics/auth/default/#the-login-required-decorator:

will use the value of redirect_field_name as its key rather than "next" (the default).

[what-the-diff]

PR Summary

• Change in 'activities.html'
  The update in members/templates/members/activities.html allows appending additional information to the URLs for the "Sign Up" and "Waitlist" buttons. This change can help keep track of data related to these actions.

• Modifications in 'activity_signup.html'
  In the members/templates/members/activity_signup.html file, the adjustments made permit embedding a query parameter to the URL for the "Login" link. It would assist in managing data or actions linked to the login process.

• Adjustment in 'header.html'
  The alteration in the members/templates/members/header.html file also incorporates a query parameter to the URLs for the "Login" link, similar to the previous update. This enhancement will support data logging or user authentication-related functionalities.

[rasmusselsmark]

Kiggede på om det også nemt kunne gøres for bruger-oprettelse, men kræver lidt mere, så har oprettet #969