countdown.js: count towards extension classes
countdown.js: copy the duplicate bells

Kinda hard to add to bells if they all point to the same arrays.
countdown.js: much undefined, wow
timetable-schema.json: update schema

+ bool useTimetable
* classes[].room is now nullable
* days now has an explicit maximum size
* int days[][].classId - negative numbers can't be unsigned :)
README.md: add link to mockup
index.html, style.css: add p#bell-current
countdown.js: show current event above countdown