Get APIv3 Specification up-to-date with its implementation

This PR brings specification and actual implementation closer together.
The specification still goes further than the implementation in some places, but it should be quite managable and understandable now. (If I did not miss anything)

This PR also temporarily disables the PATCH-Endpoint for Work Packages, as it poses a security risk in its current implementation. This should make us safe for Release 4.0 and can be fixed in a later release.