Closes #19627: Object change migrators (#19628)

* Initial work on ObjectChange data migrations

* Fix migration bug

* Add migrators for MAC address assignments

* Update reverting kwarg; allow pop() to fail

* Cross-reference MAC address migrators

* Split migrator logic across migrations

* Add missing migrator

netbox/tenancy/migrations/0018_contact_groups.py

@@ -66,3 +66,17 @@ class Migration(migrations.Migration):
             name='group',
         ),
     ]
+
+
+def oc_contact_groups(objectchange, reverting):
+    for data in (objectchange.prechange_data, objectchange.postchange_data):
+        if data is None:
+            continue
+        # Set the M2M field `groups` to a list containing the group ID
+        data['groups'] = [data['group']] if data.get('group') else []
+        data.pop('group', None)
+
+
+objectchange_migrators = {
+    'tenancy.contact': oc_contact_groups,
+}