Add month to long invoice number

[jkrbs]

There have been mistakes with duplicate invoice numbers due to user errors. As an safety net, we would like to include the month of the invoice to its number.

[jkrbs]

Ignore this request for now. I learned that invoice numbers need to be consecutive, thus I think, we need to start with R001 every month, which is incompatible with the current way of computing the invoice number https://github.com/ascii-dresden/asciii/blob/d4185c6299b6685e6887672d01923d282b02b827/src/bin/cli/subcommands/mod.rs#L350-L355

[hoodie]

habt ihr probiert ob das latex dann noch valide Pdfs generiert?