We already make use of the images 'name' key inside
skins.minerva.content.styles.images to define the full
CSS selector. We can use this approach to ship a single version of
the magnifying glass icon, rather than having 2 defined in both
skins.minerva.base.styles and skins.minerva.icons.images
Change-Id: Ie5ec06152c418c4aa1b620b3d0a904cc9517e5d0