Add preview button for comment form.

Change-Id: Ibf93bf90763c0a884f2e39492924dea48de32630
Added some explanation text.

Added text to comment form explaining that input is html. Changed Subscribe button so that it now says 'Subscribed' if the user is subscribed.

Change-Id: I8d0a25fc0c819dcfdd20c4371c1c0174240eadcb