Add missing migration for validity range

See merge request !68

Reformat all files

See merge request !67

Resolve "Link data to specific date ranges"

Closes #85

See merge request !66

Resolve "Add option to link subjects to groups"

Closes #89

See merge request !65

Fix some date/time functions and add new helper functions

See merge request !63

Add better options to get previous or next lesson period

See merge request !64

- Add properties to LessonPeriod model
- Also annotate year while annotating a week
- Fix use of negative indexes in next_lesson

Fix paths to templates. Advances AlekSIS#246

See merge request !58

Update dependency flake8-black to ^0.2.0

See merge request !60

Enable testsuite and lint

See merge request !59