new start page: ensure min width of second column and use media queries to fall back to single column layout on small displays (will certainly raise some ux issues regarding the positioning but that is to be tackled lateron - we can live with that for now), re #4525

git-svn-id: svn://develop.studip.de/studip/trunk@31667 c5b85792-5220-0410-ad23-ed5b6b8ad999