Use secret based link to sponsor-vouchers invoice on dashboard

There can be multiple managers of a sponsorship, so for all of them to
be able to view (and pay) the invoice, we need to use the secret-url
link rather tha th eone expecting the user to be the same one as the one
who created it.

Spotted by Cornelia Biacsics

Author: mhagander

template/confsponsor/sponsor.html

@@ -258,7 +258,7 @@ <h3>Pending vouchers</h3>
 </p>
 <ul>
 {%for v in pendingvouchers%}
- <li>Invoice #<a href="/invoices/{{v.invoice.id}}/">{{v.invoice.id}}</a></li>
+ <li>Invoice #<a href="/invoices/{{v.invoice.id}}/{{v.invoice.recipient_secret}}/">{{v.invoice.id}}</a></li>
 {%endfor%}
 </ul>
 {%endif%}