Close #75

To do that move some common parts to own template files

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

Close #65

Closes #59.

- Remove some unnecessary stuff from JavaScript files
- Reverse URLs on server-side
- Include JS for datepicker only if needed
- Remove JS unix date template tag and add it as an util helper instead