Add {archive_serial_number} placeholder for PAPERLESS_FILENAME_FORMAT

[ghost]

The ASN concept explained in the docs is very compelling, but I'd like to be able to export my files with the ASN in the filename, should I ever have to migrate away from paperless in the future.

I know the ASN would be on the scanned documents too, but usually handwritten and not searchable/machine-readable.

I'd prefer the placeholder to be "none" when no ASN is assigned, but whatever works best.

[jonaswinkler]

Added! I used {asn} to keep it short.