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
This should have been gone when introducing the status as link.
`IsClosed` basically means "if it has a status that is closed", which was neccessary when the status was only referenced by its name and not as a link.

In case you think it is a good idea to keep such information on the WP itself (I don't think it is), we'd at least have to discuss about naming, because a version can also be closed...