Refactor Phabricator event and last payload save code

Summary:
The Phabricator event now uses a payload property like the others
...PayloadEvent classes, and is now responsible to initialize
an instance of PhabricatorStory itself.

With a payload property, we are now able to use LastPayloadSaver
for all our payload events.

Test Plan:

• Wait a Phabricator request, check storage/logs/payload.json
• Same for a GitHub or Docker Hub request

Reviewers: dereckson

Differential Revision: http://devcentral.nasqueron.org/D122