Add a "completed" field to the Build model

- Keeps track of how many outputs have been produced
- Will not be directly editable by the user

InvenTree/build/migrations/0025_auto_20201020_1248.py

@@ -0,0 +1,24 @@
+# Generated by Django 3.0.7 on 2020-10-20 12:48
+
+import django.core.validators
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('build', '0024_auto_20201020_1144'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='build',
+            name='completed',
+            field=models.PositiveIntegerField(default=0, help_text='Number of stock items which have been completed', verbose_name='Completed items'),
+        ),
+        migrations.AlterField(
+            model_name='build',
+            name='quantity',
+            field=models.PositiveIntegerField(default=1, help_text='Number of stock items to build', validators=[django.core.validators.MinValueValidator(1)], verbose_name='Build Quantity'),
+        ),
+    ]