Fix registration for questionanswering frontend

[dschiese]

Implement a method to indicate whether a component is accessible or not

Fixed by:

• Passing the Map<Component name, status> to the Thymeleaf template instead of List
• Each request to /startquestionansweringwithtextquestion calls the cached method that returns that map
• Offline components can't be selected and appear in a darker gre

[anbo-de]

@dschiese Thanks for your insightful improvement.