dax/settings/: Re-work the username/email changing

Even though we have the username input box's set to type "email" in the
html, it seems some browsers don't enforce this, so add some checks in
the application to check for valid email addresses.

Also fix the reddening of the username input box's when there is an
error.

Signed-off-by: Andrew Clayton <andrew@dnsandmx.com>