[Lesson cancellation] Cancel a lesson is not possible

Form fields subject, teachers and room are required

changed milestone to %1.0a1

added part::frontend prio::1 type::bug + 1 deleted label

mentioned in merge request !14 (merged)

mentioned in commit 9a0ad7a8

mentioned in commit 36bcbd29

mentioned in commit 67f33093

closed via merge request !14 (merged)

mentioned in commit b70f4d1d