js updater: throttle requests, set timeout and use jquery promises, re #4323

git-svn-id: svn://develop.studip.de/studip/trunk@30033 c5b85792-5220-0410-ad23-ed5b6b8ad999
js updater: delay first request, re #4323

git-svn-id: svn://develop.studip.de/studip/trunk@30034 c5b85792-5220-0410-ad23-ed5b6b8ad999