Add deadlines for homework (or lesson-independent homework)

Homework should be separate objects. They should get

• a deadline to any lesson in future
• a text
• maybe a link or reference to additional resources

It should be possible to enter homework also for later lessons and not only for the coming lesson.

changed title from Add deadlines for homework to Add deadlines for homework (or lesson-independent homework)

changed the description

added part::frontend prio::2 source::customer workflow::discussing labels

Waiting for Alsijil model redesign

marked this issue as related to #187

added prio::3 label and removed prio::2 label