This change makes it so that when an env_role is updated to be None, the
role property on the env_role is changed to be None in addition to being
marked as deleted. This also adds in a check so that previously deleted
env_roles cannot be reassigned a role.
In the future, an `application_invitation1 will not refer to a `user` until
someone accepts the invitation; they'll only reference an
`application_role`. When a user is invited to an application, the
inviter can specify the environments the invitee should have access to.
For this to be possible, an `environment_role` should reference an
`application_role`, because no `user` entity will be known at that time.
In addition to updating all the models and domain methods necessary for
this change, this commit deletes unused code and tests that were
dependent on `environment_roles` having a `user_id` foreign key.
