Unknown errors when creating invoices

[IiroP]

Laskugeneraattori backend has quite strict rate limiting (triggered by 5+ requests in short timeframe). When sending requests that have attachments that are bigger than ~500 kB (exact amount unknown) AND rate limit has been reached, the backend terminates request before all content has been send. This can be seen when using curl:

* HTTP error before end of send, stop sending
* abort upload after having sent 524085 bytes

If this situation is triggered with Laskugeneraattori frontend, it leads to "Jotain meni pieleen" error page, with server log showing:

[joonatanaatos]

Hmm, this seems to be a problem with the invoice generator. Perhaps we could try increasing the rate limit so that it accepts requests up to, for example, 20 MB? The frontend should also prevent the request from being made if the size of the attachments exceed the limit.