Enable HTTPS & Let's Encrypt if no protocol specified for `external_url`
Right now if a user enters in their hostname for
external_url, but forgets to add a protocol we return an error. Instead of doing this, we could consider attempting to take some action instead.
We should treat this as HTTPS and attempt to use LE. In the event it fails, generate a reconfigure error. (Without this, it would have erred anyway.)
In the future we can consider falling back to HTTP in an intelligent way, so we don't attempt to request certificates on every reconfigure.