Add BodyClass and BodyId to E-mail Templates

Right now CKEditor, which is used to enter e-mail template translations, doesn't allow usage of HTML and BODY tags inside a translations. This becomes a problem, when you need to set background color of whole e-mail that will be sent.

I'm proposing to add BodyId and BodyClass fields to each E-mail Template (empty by default), which later will be:

exported/imported via language pack
inserted in place of $body_id and $body_class variables defined in e-mail template design, which is entered once during language pack editing.

To make this happen we need to change default HTML e-mail template design from "$body" to following:

<html>
	<body id="$body_id" class="$body_class">
		$body
	</body>
</html>