Make MediaViewer ignore images which are in 'noviewer' class

This includes images where the <img> element has that class
(achievable with [[File:Foo.png|class=noviewer]]) and also those
where some parent element has it.

Change-Id: I666be026828ea9ecb6e8c93d3f5ad1e3c190f81e
Mingle: https://wikimedia.mingle.thoughtworks.com/projects/multimedia/cards/511
Merge "Make MediaViewer ignore images which are in 'noviewer' class"