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 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. 

Let me know if that's a problem. I definitely think it would possible to combine them into a single endpoint. 

Neither endpoint supports a delete action, roles and preferences should simply be unset as part of an update action. 
Should we extract this out to a union all query builder? Not sure we always want a union all.

I remember the optimized classificaitons query required a union all (for what i can't remember). Does the preference query builder need the union all?