Specify patchable version

## Work Packages

* https://community.openproject.org/work_packages/17405
* https://community.openproject.org/work_packages/17406
* https://community.openproject.org/work_packages/17407

## Changes

* more properties/links version
* version linked by WP (**PATCHABLE**)
* version has a self-endpoint
* version has endpoint for projects it is available in
* all endpoints (including existing `projects/{id}/versions`) have errors for missing permissions
We can create a package for removing this but it might take some time to actually get it removed as it might already be used by the front end (@hschink, @myabc).