Add "factuur downloaden" button to generate a PDF and let the user download it

[Ellen-Wittingen]

Currently the factuur versturen knop in the invoice list in http://localhost:5000/invoices generates a pdf and then emails it to the user that the invoice is for.

We do not like this as then you do not notice an email is sent and you cannot alter it.

Instead, the button can be renamed to Factuur downloaden which would generate the pdf and download it to the computer of the person that pressed the button

[Ellen-Wittingen]

Requires #842 to be fixed first

[wilco375]

I don't think this is desired because the email also includes a link to directly pay the invoice via iDEAL and it saves the treasurer a lot of time from having to manually compose emails. We could add an extra button to just download the PDF though.