css tree: extract .collapsable so a plain .css-tree is - well - just a tree, re #4520

git-svn-id: svn://develop.studip.de/studip/trunk@31190 c5b85792-5220-0410-ad23-ed5b6b8ad999
course details: provide link to study areas, re #4520

git-svn-id: svn://develop.studip.de/studip/trunk@31191 c5b85792-5220-0410-ad23-ed5b6b8ad999