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. 
Spelling and more specific wording. I'm not sure what accessible means in this context, hence what the result should be.