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/migrations/0005_part_consumable.py

@@ -0,0 +1,18 @@
+# Generated by Django 2.2 on 2019-04-15 13:48
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('part', '0004_bomitem_note'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='part',
+            name='consumable',
+            field=models.BooleanField(default=False, help_text='Can this part be used to build other parts?'),
+        ),
+    ]