Lists of visited sessions for guest users which have not been active for
a fixed amount of time are now automatically deleted.

add empty test methods

 - add controller/service/entity/dao-functions
 - add motdlist so that users can tell the backend, which motds they have read
 - add admin account (in arsnova.properties: security.adminaccount).
 - add permission function for admin

In getSkillQuestion() a call to stubQuestions.get() contained an argument
with an incompatible class. Session will be converted into String using
toString() but this is not what was intended to do here.

In saveQuestion() the CouchDB document ID was used, but the sessions
keyword should have been used instead.

Requesting partial lists would interfere with Spring caching so
pagination is handled by the newly introduced PaginationListDecorator
in the controllers. This additionally allows to determine the total
count of items (independent from pagination parameters) which is not
possible with CouchDB.

change getQuestionsBySortOrder to fetch Questions in one single db request instead of requesting every document alone

Removed test cases because they weren't of much use after
the refactoring.