Bug 820317 - [Email] Confirmation overlay didn't sync with building-block

Unify the style of confirm dialog. Please ref these issues:
 * https://bugzilla.mozilla.org/show_bug.cgi?id=820317
 * https://bugzilla.mozilla.org/show_bug.cgi?id=821683
 * https://bugzilla.mozilla.org/show_bug.cgi?id=817609