publications: Order by reverse primay key

Change order of publications show with the  publication_list
tag, which is used to show 5 publications on the root page.

Order by reverse primary key to show last five entries.

mpicms/publications/templatetags/publication_tags.py

@@ -7,6 +7,6 @@
 @register.inclusion_tag('publications/components/list.html', takes_context=True)
 def publication_list(context):
     return {
-        'publications': Publication.objects.all()[:5],
+        'publications': Publication.objects.order_by('-pk')[:5],
         'request': context['request'],
     }