publications: Initialize sort order

Add data migration to initialize the sort order with initial values
which result in the publications being shown in the same order as
before. We change the order from '-pk' to 'sort_order'. Initialize
sort_order with -pk to get the same initial order.

mpicms/publications/migrations/0004_publication_sort_order.py

@@ -1,7 +1,10 @@
-# Generated by Django 2.2.9 on 2020-01-19 19:36
-
 from django.db import migrations, models
 
+def init_sort_order(apps, schema_editor):
+    Publication = apps.get_model('publications', 'Publication')
+    for publication in Publication.objects.all():
+        publication.sort_order = -publication.pk
+        publication.save(update_fields=['sort_order'])
 
 class Migration(migrations.Migration):
 
@@ -15,4 +18,5 @@ class Migration(migrations.Migration):
             name='sort_order',
             field=models.IntegerField(blank=True, editable=False, null=True),
         ),
+        migrations.RunPython(init_sort_order, reverse_code=migrations.RunPython.noop),
     ]