add certmagic package to host explorer frontend with https

[DylanVerstraete]

fixes #505

Example of how to integrate in Tfchain:

// Start serving frontend if there is such a folder
go func() {
	domains := []string{""}
	err = explorer.ServeFrontend(true, domains, "")
	if err != nil {
		servErrs <- err
	}
}()

[robvanmieghem]

I like the certmagic integration but not the hardcoding of LetsEncryptStagingCA Ca and the port, it also uses a different domain/port now than the explorer rest api, it would be better if they can be both served by the same port and domain+port

[robvanmieghem]

see comment

[DylanVerstraete]

Added flags to rivined:

• -c "stagingCertificate", indicates if a staging certificate should be used.
• -e "hostExplorer", indicates if we should host the explorer ourself.
• -caddy-domains list of caddy domains for our certificate.

It also now serves the frontend and backend on the same host/port.

Needs to be tested in production right now.

[GlenDC]

@LeeSmet please give a proper review of this PR ASAP.