Add "supplier_reference" field to PurchaseOrder

- This is the code that the Supplier uses for the particuarl sales order

InvenTree/order/migrations/0019_purchaseorder_supplier_reference.py

@@ -0,0 +1,18 @@
+# Generated by Django 2.2.10 on 2020-04-15 03:31
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('order', '0018_auto_20200406_0151'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='purchaseorder',
+            name='supplier_reference',
+            field=models.CharField(blank=True, help_text='Supplier order reference', max_length=64),
+        ),
+    ]