folder/received/import - Moves an existing received folder in receiver's files.

Method parameters:
id - The ID of the received folder to move.
parent_id - The ID of the parent folder.

Returns a JSON document of the following format:
<number> - The ID of the imported folder.

Expected errors:
RECEIVED_FOLDER_NOT_FOUND - A received folder with the ID doesn't exist.
PARENT_FOLDER_NOT_FOUND - A parent folder with the ID doesn't exist.