Use our own warning system instead of mw.notify

On load, the JavaScript creates a div right after
the textbox but above the submit/preview buttons. This
is near impossible to do in HTMLForm itself.

We then show/hide this message as needed, which is
less obtrusive than mw.notify popping up and going
away constantly.

Change-Id: Ic5ae117eb10801eeee13be5042196c319308c699
Merge "Use our own warning system instead of mw.notify"