Support AJAX watch in skins that use a different class structure

Support AJAX watch in skins that use a different class structure

Skipping links that are in #bodyContent or #content doesn't work for
skins like Timeless. Instead of trying to filter based on those classes,
have SkinTemplate set a data-mw=interface attribute on the <a> element,
and filter based on that. The Sanitizer prevents any data-mw attributes
being set by users, so it must have been created by MediaWiki, and
therefore safe to use.

If no link meets that criteria, then it will fallback to trying to find
a link using the old criteria of not being in #bodyContent or #content.

Bug: T173279
Change-Id: I1688a499dda40428dd514230b78ccded0d228ca3