In order to do that some parts has been refactored out in own templates.

Advances #73

- Use new template for cancellation badge (badge.html)
- Show more details like comment or specific cancellation types
- Fix some rules in substitution templates
- Fix cycle mechanism in substitutions_print.html

- Refactor build_timetable to return data for a single day
- Use lazy model imports
- Refactor lessons_col.html

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