fix(signup): block signup attempts with @firefox.com emails.

Fixes #1859
Do we need a special case here? We want this logged as an error, not a one-off event. I think validation errors get logged here: https://github.com/mozilla/fxa-content-server/blob/a6b9719af4b2145e8ab5d0c05b4e9378e1229ef9/app/scripts/views/form.js#L411