Url field fix (#3488)

* Updates for automated metadata extraction

* Update link field for StockItem model

- Increase max_length to 200 characters
- Custom migration
- Updates for InvenTreeUrlField model

* Adding unit tests

* Bug fix for metadata.py

InvenTree/stock/migrations/0082_alter_stockitem_link.py

@@ -0,0 +1,19 @@
+# Generated by Django 3.2.15 on 2022-08-07 02:38
+
+import InvenTree.fields
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('stock', '0081_auto_20220801_0044'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='stockitem',
+            name='link',
+            field=InvenTree.fields.InvenTreeURLField(blank=True, help_text='Link to external URL', max_length=200, verbose_name='External Link'),
+        ),
+    ]