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/