Add 'has_variants' and 'variant_of' field for Part

- StockItem cannot point to a part which is a template part

InvenTree/part/migrations/0003_auto_20190525_2226.py

@@ -0,0 +1,24 @@
+# Generated by Django 2.2 on 2019-05-25 12:26
+
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('part', '0002_auto_20190520_2204'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='part',
+            name='has_variants',
+            field=models.BooleanField(default=False, help_text='Is this part a template part?'),
+        ),
+        migrations.AddField(
+            model_name='part',
+            name='variant_of',
+            field=models.ForeignKey(blank=True, help_text='Is this part a variant of another part?', limit_choices_to={'active': True, 'has_variants': True}, null=True, on_delete=django.db.models.deletion.SET_NULL, related_name='variants', to='part.Part'),
+        ),
+    ]