Task #4027: getQuestionById fixes if a list of answers exist which could
not morphed by self

check if session has this question and throw unauthorized exception if
not

add type of user account
adjust test for new user account type
UserServiceTest which test persistence of the maps

This provides smaller and cleaner Interfaces for each context.

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.