HG-144: Caching for top contributors

@macbre can you take a look, please
The `$limit` is a constant set in https://github.com/Wikia/app/blob/HG-144/extensions/wikia/MercuryApi/MercuryApiController.class.php#L7  and generally should not change except when product decides to change it. If I understood correctly this counts as upper limit :smile: