Add "is_building" field to StockItem model

- This will be set to TRUE until a stock item has been completed

InvenTree/stock/migrations/0052_stockitem_is_building.py

@@ -0,0 +1,18 @@
+# Generated by Django 3.0.7 on 2020-10-04 13:12
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('stock', '0051_auto_20200928_0928'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='stockitem',
+            name='is_building',
+            field=models.BooleanField(default=False),
+        ),
+    ]