Use application_role_id on environment_roles.

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.

tests/domain/test_applications.py

@@ -128,7 +128,7 @@ def test_create_member():
     # view application AND edit application team
     assert len(member_role.permission_sets) == 2
 
-    env_roles = member_role.user.environment_roles
+    env_roles = member_role.environment_roles
     assert len(env_roles) == 1
     assert env_roles[0].environment == env1
 
@@ -165,7 +165,9 @@ def test_remove_member():
     user = UserFactory.create()
     member_role = ApplicationRoleFactory.create(application=application, user=user)
     environment = EnvironmentFactory.create(application=application)
-    environment_role = EnvironmentRoleFactory.create(user=user, environment=environment)
+    environment_role = EnvironmentRoleFactory.create(
+        application_role=member_role, environment=environment
+    )
 
     assert member_role == ApplicationRoles.get(
         user_id=user.id, application_id=application.id
@@ -181,4 +183,9 @@ def test_remove_member():
     #
     # TODO: Why does above raise NotFoundError and this returns None
     #
-    assert EnvironmentRoles.get(user_id=user.id, environment_id=environment.id) == None
+    assert (
+        EnvironmentRoles.get(
+            application_role_id=member_role.id, environment_id=environment.id
+        )
+        is None
+    )