Specify patchable category

## Work Packages

* https://community.openproject.org/work_packages/17413
* https://community.openproject.org/work_packages/17414
* https://community.openproject.org/work_packages/17415

## Changes

* Category has self-endpoint
* Category has more properties/links
* WP links to category
* WP category can be PATCHed