muting: Record DateTime when a Topic is muted.

This includes the necessary migration to add
the date_muted field to the MutedTopic class
and populates it with a hard coded value.

zerver/migrations/0262_mutedtopic_date_muted.py

@@ -0,0 +1,21 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.26 on 2020-01-17 15:26
+from __future__ import unicode_literals
+
+import datetime
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('zerver', '0261_realm_private_message_policy'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='mutedtopic',
+            name='date_muted',
+            field=models.DateTimeField(default=datetime.datetime(2020, 1, 1, 0, 0)),
+        ),
+    ]