Add support for custom fields in billing invoices

Customers (especially in Europe) need the ability to add Tax IDs, VAT IDs, and other custom fields to their invoices.

Fixes #106

util/invoice.py

@@ -25,6 +25,8 @@ def renderInvoiceToPdf(invoice, user):
 
 def renderInvoiceToHtml(invoice, user):
   """ Renders a nice HTML display for the given invoice. """
+  from endpoints.api.billing import get_invoice_fields
+
   def get_price(price):
     if not price:
       return '$0'
@@ -44,7 +46,8 @@ def renderInvoiceToHtml(invoice, user):
     'invoice': invoice,
     'invoice_date': format_date(invoice.date),
     'getPrice': get_price,
-    'getRange': get_range
+    'getRange': get_range,
+    'custom_fields': get_invoice_fields(user)[0],
   }
 
   template = env.get_template('invoice.tmpl')