add model support, an API, and a migration for Org -> Galaxy credentials

see: https://github.com/ansible/awx/issues/7813

awx/api/views/organization.py

@@ -7,6 +7,7 @@ import logging
 # Django
 from django.db.models import Count
 from django.contrib.contenttypes.models import ContentType
+from django.utils.translation import ugettext_lazy as _
 
 # AWX
 from awx.main.models import (
@@ -20,7 +21,8 @@ from awx.main.models import (
     Role,
     User,
     Team,
-    InstanceGroup
+    InstanceGroup,
+    Credential
 )
 from awx.api.generics import (
     ListCreateAPIView,
@@ -42,7 +44,8 @@ from awx.api.serializers import (
     RoleSerializer,
     NotificationTemplateSerializer,
     InstanceGroupSerializer,
-    ProjectSerializer, JobTemplateSerializer, WorkflowJobTemplateSerializer
+    ProjectSerializer, JobTemplateSerializer, WorkflowJobTemplateSerializer,
+    CredentialSerializer
 )
 from awx.api.views.mixin import (
     RelatedJobsPreventDeleteMixin,
@@ -214,6 +217,20 @@ class OrganizationInstanceGroupsList(SubListAttachDetachAPIView):
     relationship = 'instance_groups'
 
 
+class OrganizationGalaxyCredentialsList(SubListAttachDetachAPIView):
+
+    model = Credential
+    serializer_class = CredentialSerializer
+    parent_model = Organization
+    relationship = 'galaxy_credentials'
+
+    def is_valid_relation(self, parent, sub, created=False):
+        if sub.kind != 'galaxy_api_token':
+            return {'msg': _(
+                f"Credential must be a Galaxy credential, not {sub.credential_type.name}."
+            )}
+
+
 class OrganizationAccessList(ResourceAccessList):
 
     model = User # needs to be User for AccessLists's