Add 'completed_by' field to Build

- On Complete() or Cancel() user field is filled in

InvenTree/build/migrations/0009_build_completed_by.py

@@ -0,0 +1,21 @@
+# Generated by Django 2.2 on 2019-05-02 21:26
+
+from django.conf import settings
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        migrations.swappable_dependency(settings.AUTH_USER_MODEL),
+        ('build', '0008_auto_20190501_2344'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='build',
+            name='completed_by',
+            field=models.ForeignKey(blank=True, null=True, on_delete=django.db.models.deletion.SET_NULL, related_name='builds_completed', to=settings.AUTH_USER_MODEL),
+        ),
+    ]