specify patchable priorities

## Work Packages

* https://community.openproject.org/work_packages/17410
* https://community.openproject.org/work_packages/17411
* https://community.openproject.org/work_packages/17409

## Changes

* priorities have more properties
* priorities can be queried by their self-link
* priorities are linked by WPs rather than referenced by name
* WP priorities are **WRITE** -able