en.wikipedia.org has a template gala for edit notices with a whole
bunch of html framework outputted by default from
MediaWiki:Editnotice-0 (even if their underlying system has no
matches for the current page).
In the core editor from EditPage.php this isn't a problem as the
element is just idling hidden above the editor.
In the case of VisualEditor (where we have a custom delivery for
the edit notices) we don't want to say "1 notice available" on
every page, so we need to be smart and quickly walk the dom of the
notice, filter out invisible nodes, and if the resulting nodes
have no contents, ignore the notice all together.
Change-Id: I65447da8b88a9bae9c24ff155544ff66b3fe9100
