dialogs: disable dialog buttons again when either the underlying element is a link or if the associated form validates correctly; this said, don't use some weird selfmade validation - rely on the libs we have (validator.js offers a lot of functionality) and the methods that html5 webforms offer; if all else fails, poke me and i will either try to work a way around or remove the disable mechanism completely, re #4404

git-svn-id: svn://develop.studip.de/studip/trunk@32041 c5b85792-5220-0410-ad23-ed5b6b8ad999