Add a "sales_order" reference to the Build model

- If a build order is made to fulfil a sales order
- Add sales_order filtering to the Build API
- Pass initial information through to the BuildCreate view

InvenTree/build/migrations/0012_build_sales_order.py

@@ -0,0 +1,20 @@
+# Generated by Django 3.0.5 on 2020-04-24 22:51
+
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('order', '0029_auto_20200423_1042'),
+        ('build', '0011_auto_20200406_0123'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='build',
+            name='sales_order',
+            field=models.ForeignKey(blank=True, help_text='SalesOrder to which this build is allocated', null=True, on_delete=django.db.models.deletion.SET_NULL, related_name='builds', to='order.SalesOrder'),
+        ),
+    ]