DefaultRoleActor and ProjectRoleActor implementations should not hold references to the ProjectRole and Project objects

The interface for these objects has getters for the Project and ProjectRole objects. Currently we cache references to instances of them. While this works at the moment as we only actually use their immutable properties (basically the ID), it means there exists all sorts of potential for problems if the cached versions are accessed after the actual database entities have been modified.

Essentially all we need are the IDs, perhaps consider ProjectID and ProjectRoleID interfaces?