Conditionally load mem sprite

This change restricts the loading of the membership sprite even further to only those for whom the tab actually shows.

This is a hybrid of @phamann's and @jamesgorrie's suggestions here: https://github.com/guardian/frontend/pull/4636#discussion_r13587035

cc @kaelig 
Might not be included in the dist we use - https://github.com/guardian/frontend/blob/master/common/app/assets/javascripts/components/curl/dist/curl-with-js-and-domReady/curl.js