Making DuMux-course reuse compliant

DuMux course license is insufficient. It needs to be reuse compliant like DuMux. See MR=dumux:3442

assigned to @mathis

Done with !259 (merged).

changed milestone to %3.10

closed

changed milestone to %3.9

reopened

Waiting for !295 (merged) before closing again. Then all images that are not ours should be marked explicitly.

changed milestone to %3.10

assigned to @IvBu and unassigned @mathis

mentioned in merge request !295 (merged)

mentioned in commit 02b109ce

closed with merge request !295 (merged)

Done with !295 (merged).