Add "optional" field to BomItem

- Defaults to False
- Indicates that the BomItem is "optional" for a build
- Will be used in the future when calculating if a Build output is fully allocated!

InvenTree/part/migrations/0051_bomitem_optional.py

@@ -0,0 +1,18 @@
+# Generated by Django 3.0.7 on 2020-10-04 13:34
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('part', '0050_auto_20200917_2315'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='bomitem',
+            name='optional',
+            field=models.BooleanField(default=False, help_text='This BOM item is optional'),
+        ),
+    ]