Certificate pinning and 'Zero-Knowledge' warnings

This branch should have everything related to certificate pinning and the non-ZK warning.

It doesn't get us to being able to just clone-and-use, but it contains everything likely to go into 3.1.0

We should both check this over as much as possible then merge it into master and tag it 3.1.0 for release. I will perform any manual steps needed to release.

Then, once that is done, we can finish the work of getting this all automated properly.