Synchronize follower data with parent tasks in Asana bridge

Summary: Ref T2852. Setting followers (like CCs) is a separate API call, but we don't need to do anything complicated.

Test Plan: Synchronized revisions and verified the parent task got followers.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2852

Differential Revision: https://secure.phabricator.com/D6308