Avatar URL is exposed in patron export for secret donations

When user sets their donation Privacy level to "Secret" they are indicating that they don't want to be identified by the donation recipient. By exporting the `patron_avatar_url`, in `https://liberapay.com/<account_name>/patrons/export.csv`, the user might be exposed just by doing a reverse image search for such avatar. ## Impact I would hope that there is no gain in trying to deanonymise their donors, but including the avatar should not be needed and I hope it should be an easy fix. I do not wish to be compensated in any way, the reason for using HackerOne is just that I don't want to disclose the issue on Github. Thank you for your great service! :)