Automatically extract fundraising feedback from logs on our website

Offered by intrigeri on https://gitlab.tails.boum.org/tails/sysadmin/-/issues/16142#note_16380 but I never picked up the offer.

The only feedback that needs to be extracted is from the form on https://tails.boum.org/donate/canceled/.

The feedback is submitted with a GET request to "/donate/canceled/submit/" in the "?feedback" parameter.

It could then be sent by email to tails-fundraising@boum.org automatically.

During the Paris sprint, we agreed on getting this ready in 2023Q2.