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 
Just something I've started doing recently that I thought was neat was:
``` JavaScript
$('head').append($.create('link').attr({
  id: 'membership-sprite',
  rel: 'stylesheet',
  // ...
}));
```

Take it or leave it.