Add responsible/assignee to WP form schema
Test WP form payload includes assignee/responsible
Add assignee/responsible to WP form payload
Remove superfluous methods
Fix WP validation

In contrast to my expectation the contract does not trigger the WP's validation.
Thus, those validations need to be triggered separately.

This issue became visible when removing the call to WP's valid? method removed.
The call is removed because it empties the list of errors which removes those
errors added by the contract.
Test assignee/responsible update on WP form endpoint
Test API V3 WP PATCH assignee/responsible update
Implement API V3 WP PATCH assignee/responsible update
specify required permissions

[ci skip]
Always render the WP status

The WP status is always set, thus we don't need special precautions here.
Test response for invalid resource link
Throw 422 if invalid resource link is passed
Remove superfluous API V3 models
Add path helper for API V3
Adapt specs to path helper usage
Use API V3's path helper
Remove obsolete link to available statuses

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