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
I see...

the question is in which state the spec should be in the meantime... As the spec specifies most things before they are implemented, it **might** be okay to remove it here nevretheless... or remove it from the specs when implementation vanishes?