Merge branch '91-link-substitutions-and-other-week-related-stuff-not-only-to-weeks-but-also-to-years' into 'master'

Resolve "Link substitutions and other week-related stuff not only to weeks, but also to years"

Closes #91

See merge request !80

Update dependency pytest to v6

See merge request !75

Update dependency flake8-isort to v4

See merge request !79

Add some small bug fixes for queries

See merge request !78

Resolve "Use JS template strings in weekselect.js"

Closes #90

See merge request !72

Fix filter_participant to include only direct members

See merge request !76

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