add preprint related object to publication metadata

scipost_django/journals/templates/xml/publication_crossref.html

@@ -133,6 +133,13 @@
                         </ai:program>
                     </custom_metadata>
                 </crossmark>
+                <program xmlns="http://www.crossref.org/relations.xsd">
+                  {% for preprint_sub in publication.accepted_submission.thread_full %}
+                    <related_item>
+                      <intra_work_relation relationship-type="hasPreprint" identifier-type="uri">https://{{ domain }}{{ preprint_sub.get_absolute_url }}</intra_work_relation>
+                    </related_item>
+                  {% endfor %}                  
+                </program>
                 <archive_locations>
                     <archive name="CLOCKSS"></archive>
                 </archive_locations>