Free/Busy overview

Add a new page where the free/busy overview can be seen for persons and groups. Much like the timetable view in Chronos.

We still need to discuss how free/busy of groups should be handled:

• Group is considered busy if at least one member is busy
• Group is considered busy if all members are busy
• Group is considered free if all members are free
• Group is considered free if at least one member is free

For Teckids, I see scenarios with all four use cases…

It should be considered that we will need such an overview in other places, e.g. when creating a new event in the calendar. So, both backend and frontend parts should be as reusable as reasonably possible.

added part::frontend type::feature labels

added source::customer::teckids label

created branch 1231-free-busy-overview to address this issue

mentioned in merge request !1797