file/rename - Renames an existing file.

Method parameters:
id - The ID of the file to rename.
name - The new name of the file.

Returns a JSON document of the following format:
<boolean> - Always true.

Expected errors:
FILE_NOT_FOUND - A file with the ID doesn't exist.
ENTER_NAME - The new name is empty.
FILE_ALREADY_EXISTS - A file with the name already exists.