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

# Conflicts:
#	aleksis/apps/chronos/views.py

Add header box with affected teachers and groups in substitution views

See merge request !37

Move prev/next functions to TimePeriod model

Closes #65

See merge request !38

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)

Update changelog

See merge request AlekSIS/AlekSIS-App-Chronos!35

Include print view for substitutions

Closes #60

See merge request AlekSIS/AlekSIS-App-Chronos!34

Close #60

Move min_max helpers to TimePeriod as properties

Closes #59 und #43

See merge request AlekSIS/AlekSIS-App-Chronos!33

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.

Biscuit merge. Closes #53.

Closes #53

See merge request AlekSIS/AlekSIS-App-Chronos!31