Close #67

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)

Aggregates return an explicit None if there is no data to aggregate, and dict's get method
only returns teh default if the key does not exist.

Closes #43.

Closes #59.

- 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

- Add a links to quick launch
- Update url schemas for timetable to remove the need of get params
- Add i18n and reformat