I'm only concerned with validating issue dates here. In DSpace they
are generally always YYYY, YYY-MM, or YYYY-MM-DD (though in theory
they could be any valid ISO8601 format).
This also checks for cases where the date is missing and where the
metadata has specified multiple dates like "1990||1991", as this is
valid, but there is no practical value for it in our system.
For now only test fixes because they return changed data. I'm not
sure how to test the checks, because they don't return data and I
can't modify them to return boolean values without breaking the app.