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

## TODO

* clarify projects link to `versions` (available versions, not defined versions)
* specify endpoint to get projects of a version (for `availableInProjects`)
I say **not null** even though the database says null would be okay...

really? **really**?