Move membership icons

Moving the membership icons to a new sprite as it added 30kb to global.css

Icons should now only be loaded on identity pages.

cc @kaelig 
The other option I would argue - is to load them in only when requested.
Given that this is only going to be seen by [these people](https://github.com/guardian/frontend/blob/master/common/app/assets/javascripts/modules/identity/membership-tab.js#L91) perhaps that's where we load in the sprite?