require permissions to see watchers

[ci skip]
clarify 403/404 for WP scoped resources

[ci skip]
change the way watchers are added

- using links
- possibility for multiple watchers at a time
- always returns all watchers (pagination stilll possible)

Note that the term "link object" is introduced by an as of now pending
PR

[ci skip]
specify further error cases

[ci skip]