R1:3a28f86a6e03

Refactor shared code between JIRA + Asana publishers into a base class

Summary:
Ref T3687. See some discussion in D6892. The JIRA doorkeeper publisher shares a reasonable amount of code with the Asana publisher. Remedy this:

- Create `DoorkeeperFeedWorker`, where shared functionality lives (mostly related to building story context objects).
- Push responsibility for enabling/disabling a worker into this new layer, via `isEnabled()`. This allows `FeedPublisherWorker` to dynamically find and schedule…