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

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.

- 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