Prevent email clients from linkifying site domains.

As reported in [this mailing list thread](https://groups.google.com/d/msg/mozilla.dev.identity/bbQVtYLXhk8/WFgu14TG1YsJ) (plus my reply, which ~~has yet to be approved by mods~~ I accidentally didn't actually send to the list), mail clients like trying to be clever and linkifying things like URLs.

This *should* prevent that, although I couldn't test it because I couldn't figure out how to get my local Persona clone to send an email and/or display it to console.