Pretty up authentication

Hi,

this PR will be all about authentication and authorization in EvaP. The following points have already been addressed:
    - #183: access to the field is provided and it will be auto checked when the FSR bit is checked
    - #75: design is done, but needs some polish and better copy. @cyroxx please provide feedback
    - #122: done
    - #156: done
    - #191: done as by-product
    - #184: done
    - #111: done

If you think I missed an issue that falls into this topic, please raise it here. In addition to the concrete issues, the auth has been simplified and now always uses Djangos authentication framework. For the production deployment at the HPI I would recommend to configure django-auth-kerberos instead of using the Apache Basic auth facility as this allows better control over the authentication state (see #117 for an example). This generally also simplifies development as no special setup is necessary anymore.