API v3 delete user
Match resource links against API root

The API V3 routes tend to forget about their prefix ('api'). Manual tests showed
that the API root routes reliably set their prefix.
specify project reference as link

[ci skip]
Merge pull request #2331 from opf/feature/api_v3_delete_user

17654 delete user
Merge pull request #2343 from opf/fix/api_v3_failing_resource_parsing

API V3 failing resource parsing
Merge pull request #2344 from opf/fix/specify_HAL_project_reference

Specify project reference as link
remove embedded section from example

- provides no indication of embedded fields of reality
- clutters the example
- makes consistent spec-changes harder

[ci skip]
Merge pull request #2348 from opf/feature/remove_embedding_from_spec_examples

Remove optional embedded section from example responses
specify endpoint for current watchers

[ci skip]
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]
fix example link

[ci skip]
bring available watchers in line with assignees etc

- endpoint also scoped by project
- thus also returning all possible watchers, not only the ones "not
watching"

[ci skip]
clarification on 403/404 of available xxx

[ci skip]