Add staff/duty roster feature

This should probably be based on the free/busy feature. Additionally, both HNS and FSS differentiate their teachers' working hours into different categories (regular lesson, mandatory availability (for substitutions etc.), …). This should be possible with this feature too.

Analogue to the lesson planning feature, this should include some kind of time quota that can be set for every teacher.

Some kind of statistics page should be included too

added source::customer::fss source::customer::hns type::feature workflow::discussing labels

changed the description

mentioned in issue AlekSIS-App-Chronos#290

changed milestone to %2d. Implement staff lesson quotas