Don't wait for Phabricator events in the 'events' service, wait through a Taskcluster hook instead

[marco-c]

Taskcluster now supports triggering a webhook through a unique url (with a token).
We could build a new hook that is triggered by each Phabricator diff (same config as currently), creating a task associated to the diff.
That task would run the same workflow as events:

• Clone base repo, using a Taskcluster cache and robustcheckout
• Apply the diff on tip
• Push to try
• Update phabricator harbormaster state for the diff

This would resolve the issues around disk space that we have on Heroku, allowing us to do things like #2004 and #2204.
This would also allow us to make the 'events' service slimmer. If we do both this and #945, we could get rid of the 'events' service altogether.