Add order status field

- Display status field in PurchaseOrder list view

InvenTree/order/migrations/0004_purchaseorder_status.py

@@ -0,0 +1,18 @@
+# Generated by Django 2.2 on 2019-06-04 12:52
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('order', '0003_auto_20190604_2226'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='purchaseorder',
+            name='status',
+            field=models.PositiveIntegerField(choices=[(10, 'Pending'), (20, 'Placed'), (30, 'Complete'), (40, 'Cancelled'), (50, 'Lost'), (60, 'Returned')], default=10, help_text='Order status'),
+        ),
+    ]