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
What about the following cases (in both, the user making the request has the necessary permissions):
* One user is specified to become watcher and he is not allowed to be a watcher (not included in list of available watchers)
* Multiple users are specified to become watchers and some of them are not allowed to become watchers (not included in the list of available watchers)