Generalize Asana-publishable feed story objects

Summary: Ref T2852. Pulls the Differential-specific aspects of the Asana sync out of the worker. Next diff will add a publisher for Audit/Diffusion.

Test Plan: Published events, including state changes. Saw them reflected correctly in Asana.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2852

Differential Revision: https://secure.phabricator.com/D6569