Add basic spec for WorkPackageRepresenter

Signed-off-by: Alex Coles <alex@alexbcoles.com>
[wip] Represent Work Package relations in API

Add RelationModel, RelationRepresenter

Signed-off-by: Alex Coles <alex@alexbcoles.com>
Merge branch 'feature/api-work-package-embedded-relations' of github.com:opf/openproject into feature/api-work-package-embedded-relations

Conflicts:
	lib/api/v3/work_packages/work_package_representer.rb