refine authentication check to allow external authentication methods, fixes #3669

git-svn-id: svn://develop.studip.de/studip/trunk@27475 c5b85792-5220-0410-ad23-ed5b6b8ad999
refs #3565 : gui stuff

git-svn-id: svn://develop.studip.de/studip/trunk@27476 c5b85792-5220-0410-ad23-ed5b6b8ad999