Closes #10851: New staging mechanism (#10890)

* WIP

* Convert checkout() context manager to a class

* Misc cleanup

* Drop unique constraint from Change model

* Extend staging tests

* Misc cleanup

* Incorporate M2M changes

* Don't cancel wipe out creation records when an object is deleted

* Rename Change to StagedChange

* Add documentation for change staging

netbox/extras/models/__init__.py

@@ -3,9 +3,11 @@ from .configcontexts import ConfigContext, ConfigContextModel
 from .customfields import CustomField
 from .models import *
 from .search import *
+from .staging import *
 from .tags import Tag, TaggedItem
 
 __all__ = (
+    'Branch',
     'CachedValue',
     'ConfigContext',
     'ConfigContextModel',
@@ -20,6 +22,7 @@ __all__ = (
     'Report',
     'SavedFilter',
     'Script',
+    'StagedChange',
     'Tag',
     'TaggedItem',
     'Webhook',