Create a custom URL field, which allows the user-specified validators

- Ref: https://stackoverflow.com/questions/41756572/django-urlfield-with-custom-scheme
- Apply this to the URL field in the Part model

InvenTree/part/migrations/0023_auto_20190913_1401.py

@@ -0,0 +1,19 @@
+# Generated by Django 2.2.5 on 2019-09-13 14:01
+
+import InvenTree.fields
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('part', '0022_auto_20190908_0918'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='part',
+            name='URL',
+            field=InvenTree.fields.InvenTreeURLField(blank=True, help_text='Link to extenal URL'),
+        ),
+    ]