Added flashcards panel w/new api call

Currently, this is just a copy and paste version of the
AudienceQuestionPanel. My goal is to refactor the new panel
into some kind of subclass in order to reduce the overall
code size.
Copy/Pasted new PreparationQuestionPanel
Added API call for getting preparation questions
The question list views are split up into new variants
Added API calls for question variant answer count
Merge branch 'ecollab.eu' into issue-6674
Create questions with variant type
Hide buttons that are not yet implemented
Enabled preparation questions

Open Issue: Currently, depending on which button the user has
clicked, the "mode" of the QuestionPanel changes. The consequence
is that if the user clicks on the question button on the tab bar,
she will see the question mode she last selected (i.e. the
preparation questions are shown instead of the lecture questions
if the user had selected lecture questions on the InClass panel).
Implemented tasks #6675, #6676, #7485
Added methods to delete all questions, answers
Answers now support question variants
Merge remote-tracking branch 'origin/master' into issue-6674

Conflicts:
	src/main/webapp/app/view/speaker/InClass.js
	src/main/webapp/app/view/speaker/NewQuestionPanel.js
	src/main/webapp/app/view/user/InClass.js
Fixed a mysterious problem for returning users
Merge remote-tracking branch 'origin/master' into issue-6674
Merge branch 'master' into issue-6674
Removed obsolete files
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
Merge branch 'issue-6674'