Create initial models for 'Order'

- PurchaseOrder
- PurchaseOrderLineItem

(These are based on some abstract model classes)

InvenTree/build/migrations/0005_auto_20190604_2217.py

@@ -0,0 +1,19 @@
+# Generated by Django 2.2 on 2019-06-04 12:17
+
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('build', '0004_auto_20190525_2356'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='build',
+            name='part',
+            field=models.ForeignKey(help_text='Select part to build', limit_choices_to={'active': True, 'assembly': True, 'is_template': False}, on_delete=django.db.models.deletion.CASCADE, related_name='builds', to='part.Part'),
+        ),
+    ]