cleaned up / simplified image, link and table dialogs; re #4068

git-svn-id: svn://develop.studip.de/studip/trunk@29732 c5b85792-5220-0410-ad23-ed5b6b8ad999
removed media url test function; re #4068

git-svn-id: svn://develop.studip.de/studip/trunk@29733 c5b85792-5220-0410-ad23-ed5b6b8ad999