Fix image container class

Updates the image container to use one inherited from the Product
component, but things are getting a bit chaotic here. Need to seriously
consider ditching all the class shit at some point - it adds far too
much complexity to the components.

components/products/product.htm

@@ -1,4 +1,4 @@
-<div class="{{ productContainerClass }}">
+<div class="{{ mainImageContainerClass }}">
     <img class="{{ imageClass }}" alt="{{ product.title }}" src="http://placehold.it/250/250" />
 
     <div class="{{ detailContainerClass }}">