Added form validation

* Asset IDs must be unique
* Asset IDs that are legacy and unchange have no validation applied
* Asset IDs must be alphanumeric
* Values must be >=0
* Lengths, CSAs, Circuits and Cores of cables must all be >0

assets/migrations/0009_auto_20191008_2148.py

@@ -0,0 +1,18 @@
+# Generated by Django 2.0.13 on 2019-10-08 20:48
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('assets', '0008_auto_20191002_1931'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='asset',
+            name='asset_id',
+            field=models.CharField(max_length=10, unique=True),
+        ),
+    ]