remove unneccessary infobox to avoid layout glitches, fixes #3736

git-svn-id: svn://develop.studip.de/studip/trunk@27675 c5b85792-5220-0410-ad23-ed5b6b8ad999
messaging: refine error message when submitting message with empty subject, add appropriate html 'required' attribute, re #3743

git-svn-id: svn://develop.studip.de/studip/trunk@27676 c5b85792-5220-0410-ad23-ed5b6b8ad999