Set content-disposition on invoice pdf viewers

When viewing a PDF either in the user or admin interface, set the
filename using content-disposition so if the user decides to save the
file it will get a reasonable filename, and not just `document.pdf` or
similar depending on the browser.

postgresqleu/invoices/views.py

@@ -411,13 +411,15 @@ def viewinvoicepdf(request, invoiceid):
         authenticate_backend_group(request, 'Invoice managers')
 
     r = HttpResponse(content_type='application/pdf')
+    r['Content-disposition'] = 'filename={}_invoice_{}.pdf'.format(settings.INVOICE_FILENAME_PREFIX, invoice.id)
     r.write(base64.b64decode(invoice.pdf_invoice))
     return r
 
 
 def viewinvoicepdf_secret(request, invoiceid, invoicesecret):
     invoice = get_object_or_404(Invoice, pk=invoiceid, recipient_secret=invoicesecret)
     r = HttpResponse(content_type='application/pdf')
+    r['Content-disposition'] = 'filename={}_invoice_{}.pdf'.format(settings.INVOICE_FILENAME_PREFIX, invoice.id)
     r.write(base64.b64decode(invoice.pdf_invoice))
     return r
 
@@ -430,13 +432,15 @@ def viewreceipt(request, invoiceid):
         authenticate_backend_group(request, 'Invoice managers')
 
     r = HttpResponse(content_type='application/pdf')
+    r['Content-disposition'] = 'filename={}_receipt_{}.pdf'.format(settings.INVOICE_FILENAME_PREFIX, invoice.id)
     r.write(base64.b64decode(invoice.pdf_receipt))
     return r
 
 
 def viewreceipt_secret(request, invoiceid, invoicesecret):
     invoice = get_object_or_404(Invoice, pk=invoiceid, recipient_secret=invoicesecret)
     r = HttpResponse(content_type='application/pdf')
+    r['Content-disposition'] = 'filename={}_receipt_{}.pdf'.format(settings.INVOICE_FILENAME_PREFIX, invoice.id)
     r.write(base64.b64decode(invoice.pdf_receipt))
     return r
 
@@ -451,6 +455,7 @@ def viewrefundnote(request, invoiceid, refundid):
     refund = get_object_or_404(InvoiceRefund, invoice=invoiceid, pk=refundid)
 
     r = HttpResponse(content_type='application/pdf')
+    r['Content-disposition'] = 'filename={}_refund_{}.pdf'.format(settings.INVOICE_FILENAME_PREFIX, invoice.id)
     r.write(base64.b64decode(refund.refund_pdf))
     return r
 
@@ -459,6 +464,7 @@ def viewrefundnote_secret(request, invoiceid, invoicesecret, refundid):
     invoice = get_object_or_404(Invoice, pk=invoiceid, recipient_secret=invoicesecret)
     refund = get_object_or_404(InvoiceRefund, invoice=invoice, pk=refundid)
     r = HttpResponse(content_type='application/pdf')
+    r['Content-disposition'] = 'filename={}_refund_{}.pdf'.format(settings.INVOICE_FILENAME_PREFIX, invoice.id)
     r.write(base64.b64decode(refund.refund_pdf))
     return r