Add "scheduled_for_deletion" field to StockItem

- If set to True, this StockItem will be deleted (soon) by the background worker
- As deletion takes significant time, this prevents delete operations from blocking the UI

InvenTree/stock/migrations/0066_stockitem_scheduled_for_deletion.py

@@ -0,0 +1,18 @@
+# Generated by Django 3.2.4 on 2021-09-07 06:27
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('stock', '0065_auto_20210701_0509'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='stockitem',
+            name='scheduled_for_deletion',
+            field=models.BooleanField(default=False, help_text='This StockItem will be deleted by the background worker', verbose_name='Scheduled for deletion'),
+        ),
+    ]