Make email an required field in "Message composition options" screen

[cketti]

Right now it's possible to remove the email address of the default identity via Settings → [Account] → Sending mail → Composition defaults (see #8301). It's not useful to have an identity without an email address and so a lot of places expect an email address to be there. We need to stop saving changes made in the "Message composition options" screen unless there's a valid email address.