Fix wrong offset for weekdays in in_week method

See merge request !74

Currently translated at 100.0% (113 of 113 strings)

Translation: AlekSIS/AlekSIS-App-Chronos
Translate-URL: https://translate.edugit.org/projects/aleksis/aleksis-app-chronos/de/

Resolve "Don't show dates for regular plan"

Closes #88

See merge request !73

Fix migrations for validity range

See merge request !69

Update dependency isort to v5

See merge request !70

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