fix font-awesome icons for admin taxons tree

Looks like taxons tree icons are broken in my Spree 2.2.2, probably after https://github.com/spree/spree/commit/246c06619daaddc3fc106847e6b156637ade3885.
Adding some `@extend .fa;` seems to fix, because `.fa` class [is now required for all icons] (https://github.com/FortAwesome/Font-Awesome/wiki/Upgrading-from-3.2.1-to-4#new-default-syntax).
Clean trailing spaces as bonus.