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

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

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

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