A bit odd to see it load an image from a url.
$wgNoticeCloseButton = '//upload.wikimedia.org/wikipedia/foundation/2/20/CloseWindow19x19.png';
The image is part of MediaWiki core, so there's a stable url it could use to load it from the local wiki (it's in skins/common/).
But instead of loading it from the local wiki, or from bits.wikimedia.org, or Wikimedia Commons, it loads it from foundationwiki.
Let's not do that. Since skins/common has been deprecated, this should just be added to the extension.
Better yet, why is it configurable? Sounds like it shouldn't be an <img> either, but just somewhere in a CSS background-image, with @embed for automatic optimised loading via ResourceLoader (embedded in CSS, no extra http request).