Fix for StockItem creation form

- purchase_price field was required (should not be!)
- Fixed some validation issues
- Cleaned up form implementation

InvenTree/stock/migrations/0055_auto_20201117_1453.py

@@ -0,0 +1,19 @@
+# Generated by Django 3.0.7 on 2020-11-17 03:53
+
+from django.db import migrations
+import djmoney.models.fields
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('stock', '0054_remove_stockitem_build_order'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='stockitem',
+            name='purchase_price',
+            field=djmoney.models.fields.MoneyField(blank=True, decimal_places=4, default_currency='USD', help_text='Single unit purchase price at time of purchase', max_digits=19, null=True, verbose_name='Purchase Price'),
+        ),
+    ]