Add 'consumable' field to Part

- Indicates that a part can be used to make other parts
- This is the inverse of 'buildable'
- Add this field to the serializer and edit forms
- Display parameter on part info page
- BOM edit window only requests sub-parts that are marked as 'consumable'
- Also added option to edit 'units' field for part

InvenTree/part/forms.py

@@ -48,8 +48,10 @@ class EditPartForm(HelperForm):
             'URL',
             'default_location',
             'default_supplier',
+            'units',
             'minimum_stock',
             'buildable',
+            'consumable',
             'trackable',
             'purchaseable',
             'salable',