Don't synchronize Asana objects with no CCs and no responsible, non-author users

Summary: Ref T2852. Currently, we publish commits with no audit requests and reviews with no CCs or reviewers into Asana. This creates undesired notifications, so drop events which would publish an object that doesn't exist yet and has no followers or respible users.

Test Plan: Used `bin/feed republish` to publish a story about an object with no related users, saw the publish abort with the new message. Added a CC, published again, got a publish.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2852

Differential Revision: https://secure.phabricator.com/D6727