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 
Another option, which I have been thinking about recently, is curl.js our amd loader has built support for dynamically  loading css within a module. See the docs here: https://github.com/cujojs/curl i.e. you could just do:
```javascript
require('path/to/css');
```
and be done with it :smile: