Performance optimizations

Closes #107

See merge request !90

Resolve "Check (responsive) design"

Closes #94, #100 und #101

See merge request !88

Resolve "Add rules and permissions"

Closes #73 and #79

See merge request !49

 so that permissions are basing on other ones

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