Automatically GPG-sign the commits made by @JuliaRegistrator

[DilumAluthge]

It would be great if @JuliaRegistrator could automatically GPG-sign the commits it makes when it makes a PR to the General registry.

This would require the following steps:

1. Generate a new GPG keypair
2. Upload the GPG public key to the GitHub account for @JuliaRegistrator
3. Copy the GPG private key to the server on which Registrator.jl is deployed
4. Configure Registrator.jl to sign all commits with the GPG private key

[JuliaRegistrator]

Error while trying to register: Action not recognized: could