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
I now begin to wonder if we should really start allowing setting multiple watchers at once. Where does that stop? We now need to also be able to remove multiple watchers. Maybe not from the angular client but it would be consistent. But the ability for bulk operations might also be useful on other resources. Can we get away with only allowing this for assignments, and excluding creation and manipulation?