Revert "bug fix in selecting videos for exercises"

This reverts commit 632870fe4ec67904d720a8913c9edc7eb5085818.
#17: Provide a preview of videos when marking them related to exercises
update exercises
Revert "update exercises"

This reverts commit dc2f389efa52070c93365c31a429ce81b651d32c.
update exercises once again
update exercises once again
When editing exercises, show video title in related videos
When editing exercises, allow selecting multiple related videos/covered exercises
update exercises
update exercises
update exercises
update exercises
update exercises
update exercises
update exercises
update exercises
Display exercises in main catalog
'back' button on achieving exercise proficiency
prevent recursion in UserExerciseGraph's is_bounday()
update exercises