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
First of all: It is an example. All rules for error messages are defined in the errors section, the rest is up to the implementation.

However, implementors are free to return an apropriate message for each scenario (I'd like that) **or** return a single message apropriate for *every* scenario (as currently done in most cases).