Add "enabled" field to Label class

- Only list labels which are enabled
- Allows labels to be hidden / disabled without being deleted

InvenTree/label/migrations/0002_stockitemlabel_enabled.py

@@ -0,0 +1,18 @@
+# Generated by Django 3.0.7 on 2020-08-22 23:04
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('label', '0001_initial'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='stockitemlabel',
+            name='enabled',
+            field=models.BooleanField(default=True, help_text='Label template is enabled', verbose_name='Enabled'),
+        ),
+    ]