DS-1928 use the oai-identifier schema in <description>

This is a backport of #493 to dspace-4_x.

It doesn't work yet because unlike in master, in dspace-4_x there is a default namespace (xmlns="http://www.openarchives.org/OAI/2.0/oai-identifier") defined on <oai-identifier> and I couldn't figure out how to address the nodes under <oai-identifier>.