Fixed #22788 -- Ensured custom migration operations can be written.

This inspects the migration operation, and if it is not in the
django.db.migrations module, it adds the relevant imports to the
migration writer and uses the correct class name.

tests/custom_migration_operations/operations.py

@@ -0,0 +1,26 @@
+from django.db.migrations.operations.base import Operation
+
+
+class TestOperation(Operation):
+    def __init__(self):
+        pass
+
+    @property
+    def reversible(self):
+        return True
+
+    def state_forwards(self, app_label, state):
+        pass
+
+    def database_forwards(self, app_label, schema_editor, from_state, to_state):
+        pass
+
+    def state_backwards(self, app_label, state):
+        pass
+
+    def database_backwards(self, app_label, schema_editor, from_state, to_state):
+        pass
+
+
+class CreateModel(TestOperation):
+    pass