Add X-Conch-Role header to responses for single entities

[karenetheridge]

Adding the X-Conch-Role: <ro|rw|admin> header to responses for requests for single entities can let the client decide if it should invite the user to edit that entity.

Easily done with a new resultset method get_user_role, but this shouldn't be implemented until workspaces are removed from the equation, as role-based queries will become much simpler/faster then.