Resolve "Show also groups with child groups with lessons in my groups"

Closes #110

See merge request !93

In order to do that, refactor the queries to model extensions

Resolve "Fix date select on register absence form"

Closes #106

See merge request !91

Resolve "New menu item: My classes and courses"

Closes #103

See merge request !89

Currently translated at 100.0% (140 of 140 strings)

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

Do not create personal notes for cancelled lesson periods in mark_absent

See merge request !81

Merge branch '99-add-option-to-reset-personal-note-delete-at-students-view-for-class-teachers' into 'master'

Resolve "Add option to reset personal note ("delete") at students view for class teachers"

Closes #99

See merge request !86

Merge remote-tracking branch 'origin/99-add-option-to-reset-personal-note-delete-at-students-view-for-class-teachers' into 99-add-option-to-reset-personal-note-delete-at-students-view-for-class-teachers

Currently translated at 100.0% (140 of 140 strings)

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