Collapse participant list in staff evaluation form

[janno42]

The staff evaluation form lists, among other things, all participants in that evaluation. This list can get quite long, and if users want to edit other parts of the form, this results in a lot of unnecessary scrolling.

Therefore, the participants field should be collapsed as soon as it contains at least one participant, and the number of participants should be displayed in the heading of this collapse element, so that the number is visible even in the collapsed state. The number should update on field changes.