Add followers to Asana diff tasks silently

Summary: Ref T2852. Asana is launching some kind of silent follow thing today; I don't know what the API is but it's probably something like this. I'll update this to actually make the right call once the call exists, this is mostly just a placeholder so I don't forget about it.

Test Plan: None yet, this API isn't documented or live and doesn't work yet so it can't be tested.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran, moskov

Maniphest Tasks: T2852

Differential Revision: https://secure.phabricator.com/D6740