Registration receipt should be sent by email

[kittykat]

I paid for registration and accommodation for GUADEC 2017, but I have since misplaced the downloaded receipt and the website is now offline.

To avoid situations where people end up without receipts, regcfp should either email the receipt on payment. It can be in the email body or attached as a PDF.

Alternatively or in addition, it would be great if one could download receipts from previous years via the website.