2.2 Updates to Admin

I updated the icons to use Font Awesome as they weren't appearing. I also updated the classes to work with Spree 2.2's [line_items.js.coffee](https://github.com/spree/spree/blob/2-2-stable/backend/app/assets/javascripts/spree/backend/line_items.js.coffee) and removed the spree-product-assembly version to not overlap with triggers.