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 of them have linked Asana accounts, or fail permanently if they don't.
  - We check for an "ASANATASK" edge from the revision.
    - If we do not find one, we create a new task.
    - If we do find one, we load the task.
      - If we succeed, we check the chronological key of the most recent synchronized feed story ("cursor").
        - If this story is the same or newer, we update the task to synchronize it to the current state of the revision.
      - If we fail to load the task, we fail permanently ("asana task has been deleted").
  - We then publish the actual story text to the task.

Not in yet:

  - Updating followers requires separate API calls which we don't do yet.
  - No subtasks yet.
  - No sync of open/closed state.

Test Plan: {F47546}

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2852

Differential Revision: https://secure.phabricator.com/D6302