disallow users with perms 'user' to be added to institute, fixes #4076

git-svn-id: svn://develop.studip.de/studip/trunk@28914 c5b85792-5220-0410-ad23-ed5b6b8ad999
re #4063 fixes #4078

git-svn-id: svn://develop.studip.de/studip/trunk@28915 c5b85792-5220-0410-ad23-ed5b6b8ad999