documents: allow access to files for download for everyone, generalize download link creation, re #4235

git-svn-id: svn://develop.studip.de/studip/trunk@31167 c5b85792-5220-0410-ad23-ed5b6b8ad999
documents: check for presence of zip extension, re #4235

git-svn-id: svn://develop.studip.de/studip/trunk@31168 c5b85792-5220-0410-ad23-ed5b6b8ad999