Resolve "Flashcard format"
This MR adds a new content format: "Flashcard"
For this the content creation has been extended and the statistics and content presentation has been adjusted. Participants can work through the flashcards quickly.
Migration of flashcards created in v2 can be adapted after merge.
https://gitlab.com/particify/dev/foss/arsnova-backend/-/merge_requests/347 is needed for testing.
Closes #676 (closed)
Edited by Lukas Weingarten