Fix/available watchers

https://community.openproject.org/work_packages/17645
@ulferts Please excuse that I just recognize now that you might want to use [the shared example ``API V3 collection decorated``](https://github.com/opf/openproject/blob/dev/spec/lib/api/v3/support/api_v3_collection.rb). See [PriorityRepresenter's spec](https://github.com/opf/openproject/blob/dev/spec/lib/api/v3/priorities/priority_collection_representer_spec.rb) for an example.