replace all Job/JT relations with a single M2M credentials relation

Includes backwards compatibility for now-deprecated .credential,
.vault_credential, and .extra_credentials

This is a building block for multi-vault implementation and Alan's saved
launch configurations (both coming soon)

see: https://github.com/ansible/awx/issues/352
see: https://github.com/ansible/awx/issues/169

awx/main/tests/functional/models/test_activity_stream.py

@@ -54,12 +54,11 @@ class TestImplicitRolesOmitted:
         assert qs[1].operation == 'delete'
 
     @pytest.mark.django_db
-    def test_activity_stream_create_JT(self, project, inventory, credential):
+    def test_activity_stream_create_JT(self, project, inventory):
         JobTemplate.objects.create(
             name='test-jt',
             project=project,
             inventory=inventory,
-            credential=credential
         )
         qs = ActivityStream.objects.filter(job_template__isnull=False)
         assert qs.count() == 1