Implement signed repos and https

This commit relies on zipsigner-optional being included, which Daniel said he would do tomorrow. This code will build and run without zipsigner-optional, but the repo signing will not work.

This makes all local repos be proper signed repos using a locally generated certificate and private key. This also adds support for using HTTPS:// for the local repo, but this is off by default because it will trigger a prompt to users because it is a self-signed certificate. Therefore, to keep it easy, use HTTP:// by default. In the long run, I think we can add support for automatically trusting the HTTPS:// certificate when you add a local repo, but this should only happen for a local repo. For more on this topic: https://dev.guardianproject.info/issues/3406