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 
Can't get that one to work for some reason;

`GET https://profile.thegulocal.com/assets/javascripts/curl/plugin/css.js 500 (Internal Server Error) `