Install SSL cert on the donate.tpo onion

We're currently in the process of redesigning donate.torproject.org, with an aim to launch the new donate portal at the beginning of August.

As part of that work, we'd like to explore the possibility of enabling credit card payments on donate.tpo's onion site: http://yoaenchicimox2qdc47p36zm3cuclq7s7qxx6kvxqaxjodigfifljqqd.onion

At the moment the donation hangs on submission with the following console errors: tpo/web/donate-static#36

I'm going to do some digging to see if it's Stripe that's rejecting it, and for what reason – but the cert would be nice to have anyway, if possible. Thanks!