Add "target_date" for Build model

- Add "overdue" status to Build serializer

InvenTree/build/migrations/0025_build_target_date.py

@@ -0,0 +1,18 @@
+# Generated by Django 3.0.7 on 2020-12-15 12:13
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('build', '0024_auto_20201201_1023'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='build',
+            name='target_date',
+            field=models.DateField(blank=True, help_text='Target date for build completion. Build will be overdue after this date.', null=True, verbose_name='Target completion date'),
+        ),
+    ]