Start migrating the timetable view to SchoolApps' templates

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