Add unfloated thumb styling in Parsoid CSS

This styles

[[File:Wiki.png|thumb|none|An inline thumb]]

Parsoid sets the mw-halign-none class in this case (see
https://www.mediawiki.org/wiki/Parsoid/MediaWiki_DOM_spec#Images), and so far
we didn't have the CSS to actually act on that.

Change-Id: I5be19f144188a25dbe3f17c8745199cc0c7df70e