Remove `Ownable` from `Relationship` model
Add role-editing to Person list on admin page

* Adds `__sort` and `__sort_desc` parameters to resource endpoints
* Removes Authorizations widget
* Adds `user_roles` to `Person` resource representation
Merge pull request #509 from reciprocity/feature/people-authorizations

On admin page, add role-editing to People widget; remove Authorizations widget