Add 'currency' option for company

- e.g. an external supplier might have a default currency
- Adds a form input which only allows selection of allowed currency codes
- Add unit testing for currency validation

InvenTree/common/settings.py

@@ -0,0 +1,23 @@
+"""
+User-configurable settings for the common app
+"""
+
+# -*- coding: utf-8 -*-
+from __future__ import unicode_literals
+
+from moneyed import CURRENCIES
+
+from common.models import InvenTreeSetting
+
+
+def currency_code_default():
+    """
+    Returns the default currency code (or USD if not specified)
+    """
+
+    code = InvenTreeSetting.get_setting('INVENTREE_DEFAULT_CURRENCY')
+
+    if code not in CURRENCIES:
+        code = 'USD'
+    
+    return code