Change email verification link from http:// to https://

[benfrancis]

Reported by @jaller94 on Matrix, the email verification link is currently hard coded as plain HTTP but then redirects to HTTPS, which may cause security warnings in some web browsers.

We should change it to https (or make it configurable, but not sure why anyone would want this particular feature over plain HTTP!).

This appears to be set in https://github.com/WebThingsIO/registration_server/blob/master/src/routes.rs#L619

[jaller94]

I received the following e-mail text:

Hello,
Welcome to your WebThings Gateway! To confirm your email address, navigate to http://api.webthings.io/verifyemail?s=6552ce05-e178-41f4-9dfb-f3853836631b.
Your gateway can be accessed at https://redacted.webthings.io/.

My e-mail client is K-9 Mail 14.0 and my browser is Firefox 146.0 with HTTPS-Mode set to "On in all tabs" – both on Android.