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

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

awx/main/models/organization.py

@@ -45,6 +45,12 @@ class Organization(CommonModel, NotificationFieldsModel, ResourceMixin, CustomVi
         blank=True,
         through='OrganizationInstanceGroupMembership'
     )
+    galaxy_credentials = OrderedManyToManyField(
+        'Credential',
+        blank=True,
+        through='OrganizationGalaxyCredentialMembership',
+        related_name='%(class)s_galaxy_credentials'
+    )
     max_hosts = models.PositiveIntegerField(
         blank=True,
         default=0,
@@ -108,6 +114,23 @@ class Organization(CommonModel, NotificationFieldsModel, ResourceMixin, CustomVi
         return UnifiedJob.objects.non_polymorphic().filter(organization=self)
 
 
+class OrganizationGalaxyCredentialMembership(models.Model):
+
+    organization = models.ForeignKey(
+        'Organization',
+        on_delete=models.CASCADE
+    )
+    credential = models.ForeignKey(
+        'Credential',
+        on_delete=models.CASCADE
+    )
+    position = models.PositiveIntegerField(
+        null=True,
+        default=None,
+        db_index=True,
+    )
+
+
 class Team(CommonModelNameNotUnique, ResourceMixin):
     '''
     A team is a group of users that work on common projects.