These invoices are based on the data in members.csv which in turn was
added based on the invoices we sent last year. If member co-ops get
back to us and say their details have changed or they're not able to
contribute this year, we can update the CSV and re-generate the
invoices.
I've decided to commit the generated HTML rather than the PDF, as the
latter changes when I re-run the script even if the data hasn't
changed. If we need to update any data, this should make the diffs
more obvious, but of course it'll make it harder for anyone to just
grab the PDFs from the repo without installing the toolchain. For now,
it's me doing it so we can cross that bridge at a later date.
