Build part change fix (#5808)

* Add DiffMixin class

- Allows us to compute "diffs" against items stored in the database, when they are being updated

* Prevent build order part from being changed after creation

* Remove "part" field from buildorder edit form

* Add unit test

InvenTree/templates/js/translated/build.js

@@ -130,6 +130,9 @@ function editBuildOrder(pk) {
 
     var fields = buildFormFields();
 
+    // Cannot edit "part" field after creation
+    delete fields['part'];
+
     constructForm(`{% url "api-build-list" %}${pk}/`, {
         fields: fields,
         reload: true,