API specification.
Changed StatisticsController to allow requests without a trailing slash
to be consistent with the other **/statistics paths.

This allows expecting those exceptions in test classes.

This change is needed due to client developer restriction not to round
returned value on client side but on server side, because this would
change sth.

This provides smaller and cleaner Interfaces for each context.

All relevant information about users (e.g. session mebership) are
accessible via UserService.
The method IUserService::getCurrentUser provides information about
the actual logged in user. If no user is logged in, this method will
throw a UnauthorizedException which will end up in 401 HTTP error.

All feedback methods are available in class FeedbackController, all
question methods are available in class QuestionController.

None of the URIs are changed!

Added second URI for method QuestionController::getSkillQuestions that
matches common URI rules in ARSnova:
"/session/{sessionkey}/skillquestions"
Both, the old an the new URI can be used equally.