forced ie7 into single column view to avoid need for box-sizing polyfill

@triblondon 

I'm thinking of maybe removing the following too as they encourage non-responsive products:

* `$o-grid-fixed-layout-selector: null !default` overrides choice of useragents (normally ie8) with a custom selector
* `$o-grid-is-fixed-desktop`: \[false\] Forces the site to always use the large layout
Could we rename this to something like `o-grid-useragents-fixed-to-large-layout`?