Add a default_location to the PartCategory model

- If a part does not specify a default location, look at the default location of  the category

InvenTree/part/migrations/0015_partcategory_default_location.py

@@ -0,0 +1,20 @@
+# Generated by Django 2.2 on 2019-05-04 08:57
+
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('stock', '0013_remove_stockitem_uuid'),
+        ('part', '0014_auto_20190502_2039'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='partcategory',
+            name='default_location',
+            field=models.ForeignKey(blank=True, help_text='Default location for parts in this category', null=True, on_delete=django.db.models.deletion.SET_NULL, related_name='default_categories', to='stock.StockLocation'),
+        ),
+    ]