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

docs/models/extras/branch.md

@@ -0,0 +1,13 @@
+# Branches
+
+A branch is a collection of related [staged changes](./stagedchange.md) that have been prepared for merging into the active database. A branch can be mered by executing its `commit()` method. Deleting a branch will delete all its related changes.
+
+## Fields
+
+### Name
+
+The branch's name.
+
+### User
+
+The user to which the branch belongs (optional).

docs/models/extras/stagedchange.md

@@ -0,0 +1,26 @@
+# Staged Changes
+
+A staged change represents the creation of a new object or the modification or deletion of an existing object to be performed at some future point. Each change must be assigned to a [branch](./branch.md).
+
+Changes can be applied individually via the `apply()` method, however it is recommended to apply changes in bulk using the parent branch's `commit()` method.
+
+## Fields
+
+!!! warning
+    Staged changes are not typically created or manipulated directly, but rather effected through the use of the [`checkout()`](../../plugins/development/staged-changes.md) context manager.
+
+### Branch
+
+The [branch](./branch.md) to which this change belongs.
+
+### Action
+
+The type of action this change represents: `create`, `update`, or `delete`.
+
+### Object
+
+A generic foreign key referencing the existing object to which this change applies.
+
+### Data
+
+JSON representation of the changes being made to the object (not applicable for deletions).