Replace wiki link with relative repository link.

* skillquestions -> contentlists
* lecturequestions -> lecturecontentlists
* preparationquestions -> preparationcontentlists
* flashcardquestions -> flashcardcontentlists
* questions -> contents
* answers -> answerlists
* learningprogress -> score

* Prefer ids over "foreign" domain objects.
* Consistently use final keyword for implementation parameters and
  variables.
* As a consequence, session id is used instead of the object as cache
  key for `skillquestions`, `lecturequestions`, `preparationquestions`
  and`flashcardquestions` caches.
* Rename occurances of `questionId` to `contentId`.