Load preparation questions

Introduced a new CouchDB view method, and a new field to divide
'skill_questions' into preparation, lecture, and flashcard
variants. The former question type 'flashcard' will be removed
soon.
Finalized methods to get new question variants along w/count
Added methods to count answers of question variants

Note that this is currently untested. :o
Merge branch 'master' into issue-6674
Create question variants, added several question filter functions
Merge branch 'master' into issue-6674

Conflicts:
	src/main/webapp/WEB-INF/spring/spring-main.xml
Fix for #7654: Count unanswered variant questions
Add CouchDB default username and password to configuration example
Merge remote-tracking branch 'origin/issue-6674' into issue-6674
Fix deletion of free text answers (refs #7880)

There was a bug in the session retrieval code causing an exception.
Merge branch 'master' into issue-6674
Merge branch 'master' into issue-6674
Delete all interposed questions with one request
Merge branch 'master' into issue-6674

Conflicts:
	src/main/java/de/thm/arsnova/dao/CouchDBDao.java
	src/main/java/de/thm/arsnova/dao/IDatabaseDao.java
	src/main/java/de/thm/arsnova/services/IQuestionService.java
	src/main/java/de/thm/arsnova/services/QuestionService.java
	src/test/java/de/thm/arsnova/dao/StubDatabaseDao.java
Merge remote-tracking branch 'origin/issue-6674' into issue-6674
Removed unused methods
Fixed #6620: Server API to open/close all questions of a session
Fixed #6618: Allow clients to delete all answers with one request
Changed exception type to match the default inside this class
Removed unused field