Fetch and compare followers by id in batches of 100

* The followers call was hitting this https://dev.twitter.com/docs/api/1.1/get/followers/list which fetches in batches of 20 - for users with hundreds of followers, this can cause rate limits to kick in pretty fast
* This PR changes the call such that it now: fetches a list of follower _ids_ (batches of 100), checks the intersection, and then fetches user data for those ids in one call.