Remove need for `GATEKEEPER_URL` config setting

`GATEKEEPER_URL` is needed to construct the authorize URL when entering a passcode. `GATEKEEPER_URL` is the protocol and domain name (e.g. base URL) of the service. I could get this info from the request's host header, but that may open the service to a host header attack vector. May not be worth the effort relative to the potential security vulnerability.