Close #68

Refactor out a lot of stuff used by TimetableWidget and my_timetable both to models and model extensions

Close #65

- On/off switchable via constance
- Included in substitutions.html and substitutions_print.html
- Add extra QuerySet functions (affected_lessons, affected_groups, affected_teachers)

Close #60

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

Reason: Teachers can have primary groups, too

- Move min/max functions and get_next_relevant_day to min_max.py
- Remove boilerplate code for remapping 0-sundays

Because leading underscore is reserved for private class members

- Show substitutions only for one day instead for one week

Current status: Time and weekday headings are shown

- Add custom stylesheets from SchoolApps to timetable.css
- Customize plan.html for new data structure
- Change sorting of lessons from column-based to row-based system to support CSS and HTML structures better
- Add plan.html instead of tt_week.html to timetable view
- Add short variants of weekday names