Fix "Print invoice" link not working

[loan-laux]

Resolves #348
Impact: critical
Type: bugfix

Issue

When viewing the details of an order, the "Print invoice" link doesn't work because the link's URL doesn't include the current shop ID prefix.

Solution

Include the current shop ID prefix in the link's URL.

Testing

1. Open an order's details page.
2. Click "Print invoice".
3. Notice that the printable invoice opens fine.
4. Click "Back" to go back to the order's details page.
5. Notice that the "Back" button works fine.