Added pdf/a2-b standard compliance for pdf mail submission

requested review from @jrossel

assigned to @nniklas

Not sure if that is enough. I've only had a brief look at it, but there seems to be quite a bit more to the change. We should check this: https://www.mathstat.dal.ca/~selinger/pdfa/.

At least this sounds promising:

If your pdfTeX version is at least 1.40.15 (see Step 1: Prerequisites), then your generated document should already comply with the PDF/A-1b standard at this point.