Export the categories as property in iCalendar and map existing internal categories to imported events, fixes #3931

git-svn-id: svn://develop.studip.de/studip/trunk@28230 c5b85792-5220-0410-ad23-ed5b6b8ad999
send notification messages for new forum threads if e-mail setting is *always* or *if requested by sender*, refs #3590

git-svn-id: svn://develop.studip.de/studip/trunk@28231 c5b85792-5220-0410-ad23-ed5b6b8ad999
refs #3929
encoding made case insensitive

git-svn-id: svn://develop.studip.de/studip/trunk@28232 c5b85792-5220-0410-ad23-ed5b6b8ad999