Commercial comps tweaks2

This tidies up a load of small issues with Masterclasses and Soulmates.

Widths are now calculated when using 1/3 & 2/3, makes it far better than 33% & 66%.

Mixins made for duplicated code used in Masterclasses component.
I'm assuming that's the case as when looking here http://caniuse.com/svg the same Android browsers don't support SVG as background-cover. 

Would be great if there was a list of browsers that get the svg class. (Having done a quick google I get to this page https://github.com/guardian/frontend/blob/master/docs/browser-support-principles.md - by you!) that links out to this dead page https://frontend.gutools.co.uk/analytics/browsers.