pedagogy: don't update uv comment publish date at each save

pedagogy/migrations/0001_initial.py

@@ -1,5 +1,5 @@
 # -*- coding: utf-8 -*-
-# Generated by Django 1.11.20 on 2019-06-18 15:02
+# Generated by Django 1.11.20 on 2019-06-18 20:07
 from __future__ import unicode_literals
 
 from django.conf import settings
@@ -248,7 +248,7 @@ class Migration(migrations.Migration):
                 ),
                 (
                     "publish_date",
-                    models.DateTimeField(auto_now=True, verbose_name="publish date"),
+                    models.DateTimeField(blank=True, verbose_name="publish date"),
                 ),
                 (
                     "author",