Specify 'default' functions for 'reference' field in SalesOrder and PurchaseOrder

- Refactor CreatePurchaseOrder form
- Some migrations

InvenTree/company/migrations/0040_alter_company_currency.py

@@ -0,0 +1,20 @@
+# Generated by Django 3.2.4 on 2021-07-02 13:21
+
+import InvenTree.validators
+import common.settings
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('company', '0039_auto_20210701_0509'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='company',
+            name='currency',
+            field=models.CharField(blank=True, default=common.settings.currency_code_default, help_text='Default currency used for this company', max_length=3, validators=[InvenTree.validators.validate_currency_code], verbose_name='Currency'),
+        ),
+    ]