Resolve "Improve display of groups in timetable views"

Closes #71

See merge request !46

- Add CSS fix for correct style of text collapsibles in timetables

Advances #71

if there is only one group and this group has parent groups

Advance #71

Improve handling of child/parent groups

Closes #54

See merge request !45

Close #54

Make missing models extensible

See merge request !44

Resolve "Models for advanced features from SchoolApps (imported from Untis)"

Closes #58

See merge request !39

cancelled_for_teachers, comment

- timefrom/timeto from Event
- periodfrom from Exam

Show dates in column headings in weekly timetable view

Closes #68

See merge request !43

Show announcements in timetable views

Closes #63

See merge request !41

Close #68

at_date → on_date
in_days → within_days

Drop bootstrap4 dependency. Advances AlekSIS#197

See merge request !42