Add preferences and roles endpoints

This adds end points for setting and retrieving user roles and user preferences for projects. 

While both are stored on the same model, I decided to separate them into two endpoints to simplify the handling access control. The `/project_roles` endpoint allows a project owner to set roles for users, and allows users to see what roles they have been assigned. The `/project_preferences` endpoint allows users to update and retrieve their own preferences for a project. 

Neither endpoint supports a delete action, roles and preferences should simply be unset as part of an update action.