We need to sign the OpenFeedback POST requests to prevent spam reports
coming from anywhere other than our application. I doubt this will ever be
much of a problem (famous last words), but it'll be nice to have just in case.

Like AquaticPrime, the developer inserts a private key into their app, and
we use it to generate a signature which can be sent along with all the
other feedback data. The web server can then accept or reject the feedback
based on the signature.