Fixed issue Task #8530. Guest sessions are now shown properly.
Replaced 'Session' label with 'Start' (Task #8532)
userTabPanel is now shown directly as student. (Task #8531)
Renamed MathJaxMessageBox to CustomMessageBox,
overridden confirm method of Ext.MessageBox (Task #8535)
changed order of confirm messagebox buttons. (Task #8535)
Fixed #8533, #8534, #8537: fixes for tab bar icons

The info panel just redirects to the manual, which contains
imprint and FAQs. All other entries are now located in a new
diagnosis panel.

The "home" panel icon is now hidden at all times. This fixes
the appearance of the "home" icon before joining any session.
Merge branch 'master' into issue-6674