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
@NobodysNightmare this does not take the visibility of the projects into account. The user might not be able to see the ```definingProject```. In that case it might be null/or at least not something the user can follow, right?