Harmonize watchers endpoints

## OpenProject Work Packages

* https://community.openproject.org/work_packages/17755
* includes non WP specifications that harmonize the API

## Content

* new endpoint for current watchers of a work package
* clarification on when to return 404 and 403 for some endpoints
    * this change *should* not affect the actual implementation, however I did not verify that
* allow adding multiple watchers at once
* endpoint for available watchers is now also placed under a project scope

## Changes to existing endpoints

* watchers are linked in Work Package
* the `watch` link of the work package has an updated `data` section
* embedding format of watchers changes (real collection)
* add watchers now accepts a new request format
* add watchers now returns the list of current watchers
* the available watchers endpoint was moved into a per project scope
https://github.com/opf/openproject/pull/2327 already adds such a section.
We, meaning I, should add the `payload` there.

Indeed I was thinking about a good name, but couldn't come up with something as beautiful as `payload`.
I will use that.