The JS logic didn't account for edge cases like scrolling
to the inserted comment, or attaching the comment in the right sorting logic,
on the right pagination page. It also doesn't show any "loading" indication,
so is bad usability for the majority of users. A standard form submission
does the same job better in this case. Note that this doesn't affect
the ability to moderate/delete comments via ajax.
Replaced 'use_ajax_commenting' with 'include_js',
since its important for behaviour other than comment submission itself,
e.g. showing previews (doesn't work without JS)
FIX Also fix up preview to only output the comment content rather than the whole template.
FIX Hide preview after posting comment.
API Move AllowHtml to field to prevent issues with altering Html configuration after comments have been posted.
FIX If moderation is turned on for commenting, still render comments in preview mode.
