Backporting fix for DS-1928 in PR#493 to DSpace 4.x

See PR #493 and https://jira.duraspace.org/browse/DS-1928

Because the style/theme (style.xsl) for OAI-PMH changed significantly between DSpace 4 and 5, the fix in PR #493 had to be backported to 'dspace-4_x' instead of being cherry-picked.

I've tested this backport. Seems to work fine, and it's really only the style.xsl which requires different changes.