Let new publications be first to display

Overwrite wagtailorderable.models.Orderable.save() to initialize new
objects with a lower sort order than existing objects, so that the new
objects appear at the top of the list.

mpicms/publications/models.py

@@ -10,9 +10,16 @@
 
 from modelcluster.models import ClusterableModel
 
+class LatestFirstOrderable(Orderable):
+    def save(self, *args, **kwargs):
+        if self.pk is None:
+            self.sort_order = self.__class__.objects.aggregate(models.Min('sort_order'))['sort_order__min'] - 1
+        super(Orderable, self).save(*args, **kwargs)
+    class Meta:
+        abstract = True
 
 @register_snippet
-class Publication(index.Indexed, ClusterableModel, Orderable):
+class Publication(index.Indexed, ClusterableModel, LatestFirstOrderable):
     title = RichTextField(_('title'), features=['bold', 'italic', 'link'])
     groups = RichTextField(_('groups'), features=['bold', 'italic', 'link'], blank=True)
     authors = RichTextField(_('authors'), features=['bold', 'italic', 'link'], blank=True)