Adding specific permissions for viewers for DNEnvironment.

Allows setting of a viewer group or specific viewer users for a
deployment environment. This is useful for locking off certain
environments from being viewable (e.g. only certain users can see some
hidden environments)

Because this could potentially stop existing users from being able to
view environments, CopyProjectViewersToEnvironmentViewersTest should be
run to copy existing viewer groups from DNProject to DNEnvironments
inside that project.