Store a reference to the generated PDF

Problem / Motivation

When a PDF file is generated for an invoice, we should store a reference to it instead of trying to figure out whether it exists via a query.

The patch from #3106014: Allow altering the invoice PDF filename via event subscribers is adding a field, but we're missing getters / setters as well as tests.

Also, the "challenging" part is that the field wasn't present before, which means if we're always trying get the file from the reference field, it won't work for existing invoices.

I'm not sure about the naming for that new field though, the patch from #3106014: Allow altering the invoice PDF filename via event subscribers went with "invoice_file".

To sum up, we need:

• A file reference field, translatable
• Getters/setters for that field (probably getFile(), setFile())
• Update savePrintable() to call a getFile() method that returns the file reference field if not empty, and fallback to the logic that we have today (In case a file is found, we need to set the reference.

Open questions:

• Should we delete the reference to the file on Invoice::postDelete()? Or do we always want to keep the files?