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 
You'll need to add the path to the curl config for it to map to the hashed file correctly, such as this example:
https://github.com/guardian/frontend/blob/master/common/app/views/fragments/javaScriptLaterSteps.scala.html#L35