Advanced data in timetable views

Closes #72 and #73

See merge request !47

Replace ugettext_lazy by gettext_lazy. Advances AlekSIS#224

See merge request !49

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