Rework DjangoStaticFileHandler (fixes #1034)
Rework DjangoStaticFileHandler (fixes #1034)

Fixes multiple issues
 - Unicode errors when path contained non-ascii characters

 - 403 errors when the current working directory was not a parent
   directory of the static files

 - "API abuse" super() implementation of the get() method was passed a
   filesystem path instead of the url-path it was expecting

 - The above could also lead to 403 errors on case-insensitive file
   systems due to the way the base-class resolved and validated paths
   with respect to self.root

rolandgeider/OpenSlides,ostcar/OpenSlides,CatoTH/OpenSlides,FinnStutzenstein/OpenSlides,OpenSlides/OpenSlides,boehlke/OpenSlides,jwinzer/OpenSlides,tsiegleauq/OpenSlides,jwinzer/OpenSlides,normanjaeckel/OpenSlides,FinnStutzenstein/OpenSlides,emanuelschuetze/OpenSlides,rolandgeider/OpenSlides,boehlke/OpenSlides,jwinzer/OpenSlides,CatoTH/OpenSlides,ostcar/OpenSlides,jwinzer/OpenSlides,FinnStutzenstein/OpenSlides,rolandgeider/OpenSlides,normanjaeckel/OpenSlides,tsiegleauq/OpenSlides,emanuelschuetze/OpenSlides,ostcar/OpenSlides,FinnStutzenstein/OpenSlides,normanjaeckel/OpenSlides,normanjaeckel/OpenSlides,CatoTH/OpenSlides,boehlke/OpenSlides,jwinzer/OpenSlides,tsiegleauq/OpenSlides,boehlke/OpenSlides,CatoTH/OpenSlides,emanuelschuetze/OpenSlides,emanuelschuetze/OpenSlides,OpenSlides/OpenSlides