Comment form email address validation

[wturrell]

I note for anonymous users there's no validation of the email address on the comment form – currently it's just a check that all input/selected/textarea fields with a HTML5 'required' attribute have a non-empty value.

I think it would be preferable to either match the standard format using a JS regex, or fallback to the email validation supported by HTML5 browsers, which is currently overridden.