R1:5a6044dbaa09

Initial Asana sync for Differential

Summary:
Ref T2852. This is highly incomplete but seems structurally sound. Some additional context is available in the Google doc.

- Add a workspace ID configuration. Without it, nothing else activates.
- Add a worker which reacts to feed stories.
- Feed stories about things which aren't Differential objects are ignored.
- We load the revision, or fail permanently if we can't.
- We get all the related user PHIDs (author, reviewers, CCs).
- We check if any…