models: Migration of Message.pub_date to date_sent, part 2.

Fixes #1727.

With the server down, apply migrations 0245 and 0246. 0246 will remove
the pub_date column, so it's essential that the previous migrations
ran correctly to copy data before running this.

zerver/migrations/0246_message_date_sent_finalize_part2.py

@@ -0,0 +1,45 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.23 on 2019-08-28 19:48
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('zerver', '0245_message_date_sent_finalize_part1'),
+    ]
+
+    operations = [
+        # Until now, date_sent was in ArchivedMessage only for the sake of keeping the model
+        # compatible with Message.
+        #  We can now remove it, and rename pub_date to date_sent to have this column
+        # set correctly for all existing rows.
+        migrations.RemoveField(
+            model_name='archivedmessage',
+            name='date_sent',
+        ),
+        migrations.RenameField(
+            model_name='archivedmessage',
+            old_name='pub_date',
+            new_name='date_sent',
+        ),
+        # All the below AlterField does is change verbose_name, which doesn't even generate any SQL,
+        # it's just a purely-Django attribute.
+        migrations.AlterField(
+            model_name='archivedmessage',
+            name='date_sent',
+            field=models.DateTimeField(db_index=True, verbose_name='date sent'),
+        ),
+        # Django doesn't rename the index when renaming a column, which can be confusing
+        # for someone inspecting the table in the future who's not aware of the old name.
+        # We should rename appropriately here.
+        migrations.RunSQL("""
+        ALTER INDEX IF EXISTS zerver_archivedmessage_pub_date_509062c8 RENAME TO zerver_archivedmessage_date_sent_509062c8
+        """),
+        migrations.RemoveField(
+            model_name='message',
+            name='pub_date',
+        ),
+    ]