billing: mention payment date on bill

- in PDF version
- in HTML version

fix #94

coin/billing/templates/billing/invoice.html

@@ -37,6 +37,10 @@
     </tbody>
 </table>
 
+<p>
+  Facture à payer avant le {{ invoice.date_due }}.
+</p>
+
 <h3>Règlement</h3>
 
 {% if invoice.payments.exists %}

coin/billing/templates/billing/invoice_pdf.html

@@ -203,6 +203,9 @@
        
     </tbody>
   </table>
+  <p>
+    Facture à payer avant le {{ invoice.date_due }}.
+  </p>
 
   <div id="paiements">
   {% include "billing/payment_howto.html" %}