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/models/test_environments.py

@@ -3,13 +3,7 @@ from atst.models.environment_role import CSPRole
 from atst.domain.environments import Environments
 from atst.domain.applications import Applications
 
-from tests.factories import (
-    PortfolioFactory,
-    UserFactory,
-    EnvironmentFactory,
-    ApplicationFactory,
-    ApplicationRoleFactory,
-)
+from tests.factories import *
 
 
 def test_add_user_to_environment():
@@ -22,9 +16,13 @@ def test_add_user_to_environment():
     )
     dev_environment = application.environments[0]
 
-    ApplicationRoleFactory.create(user=developer, application=application)
-    dev_environment = Environments.add_member(
-        dev_environment, developer, CSPRole.BASIC_ACCESS.value
+    application_role = ApplicationRoleFactory.create(
+        user=developer, application=application
+    )
+    EnvironmentRoleFactory.create(
+        application_role=application_role,
+        environment=dev_environment,
+        role=CSPRole.BASIC_ACCESS.value,
     )
     assert developer in dev_environment.users