ApiSandbox: Better handling of parsed messages

ApiSandbox: Better handling of parsed messages

There are several places where we're parsing a message to an HTML string
and passing it into ooui in a way that it'll interpret it as text. There
are more where we're handling it properly, but by parsing HTML strings
outselves instead of letting mw.Message do it.

So, let's add a Util function that will parse a message using
.parseDom() and apply our fixups.

Change-Id: I1f71916ac2fb3567c2fa2dffc64c4c8c91050ee9