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/routes/applications/test_team.py

@@ -94,7 +94,9 @@ def test_update_team_environment_roles(client, user_session):
     )
     environment = EnvironmentFactory.create(application=application)
     env_role = EnvironmentRoleFactory.create(
-        user=app_role.user, environment=environment, role=CSPRole.NETWORK_ADMIN.value
+        application_role=app_role,
+        environment=environment,
+        role=CSPRole.NETWORK_ADMIN.value,
     )
     user_session(owner)
     response = client.post(
@@ -121,7 +123,9 @@ def test_update_team_revoke_environment_access(client, user_session, db, session
     )
     environment = EnvironmentFactory.create(application=application)
     env_role = EnvironmentRoleFactory.create(
-        user=app_role.user, environment=environment, role=CSPRole.BASIC_ACCESS.value
+        application_role=app_role,
+        environment=environment,
+        role=CSPRole.BASIC_ACCESS.value,
     )
     user_session(owner)
     response = client.post(
@@ -177,8 +181,11 @@ def test_create_member(client, user_session):
     assert response.location == expected_url
     assert len(user.application_roles) == 1
     assert user.application_roles[0].application == application
-    assert len(user.environment_roles) == 1
-    assert user.environment_roles[0].environment == env
+    environment_roles = [
+        er for ar in user.application_roles for er in ar.environment_roles
+    ]
+    assert len(environment_roles) == 1
+    assert environment_roles[0].environment == env
 
 
 def test_remove_member_success(client, user_session):