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 doorkeeper publishers, so third parties can add additional doorkeeper publishers.
• Some general cleanup/documentation.

Test Plan: Used bin/feed republish to republish stories about objects with JIRA and Asana links. Verified that doorkeeper publishers activated properly, made calls, and published events into the remote systems.

Reviewers: btrahan, akopanev22

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3687

Differential Revision: https://secure.phabricator.com/D6906