Improve the 'creation date' for order

- Automatically set on first save

InvenTree/order/migrations/0012_auto_20190617_1943.py

@@ -0,0 +1,18 @@
+# Generated by Django 2.2.2 on 2019-06-17 09:43
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('order', '0011_auto_20190615_1928'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='purchaseorder',
+            name='creation_date',
+            field=models.DateField(blank=True, null=True),
+        ),
+    ]