Make archived PDF compatible with PDF/A

[nuest]

Background: https://en.wikipedia.org/wiki/PDF/A

Should be possible with ghostscript, see https://stackoverflow.com/questions/1659147/how-to-use-ghostscript-to-convert-pdf-to-pdf-a-or-pdf-x

For validation, use http://docs.verapdf.org/cli/validation/

[nuest]

@tnier01 This has been open for a long while - and every student should get a little maintenance task from time to time - would you mind taking a go at this if you need a break from OJS?