Django migrations doesn't account for PostgreSQL completely

This was a weird bug to run into.  Basically I changed a CharField into
a ForeignKey field and ran `makemigrations` to get the job done.
However, rather than doing a `RemoveField` and an `AddField`, migrations
created a single `AlterField` which worked just fine in SQLite, but blew
up in PostgreSQL with:

    psycopg2.ProgrammingError: operator class "varchar_pattern_ops" does
    not accept data type integer

The fix was to rewrite the single migration into the two separate steps.

src/documents/migrations/0003_sender.py

@@ -47,7 +47,11 @@ class Migration(migrations.Migration):
             ],
         ),
         migrations.RunPython(move_sender_strings_to_sender_model),
-        migrations.AlterField(
+        migrations.RemoveField(
+            model_name='document',
+            name='sender',
+        ),
+        migrations.AddField(
             model_name='document',
             name='sender',
             field=models.ForeignKey(blank=True, on_delete=django.db.models.deletion.CASCADE, to='documents.Sender'),