Implement artifact release for Harbormaster

Summary: Resolves T5836.  This automatically releases artifacts when Harbormaster builds finish (either passing or failing).  This allows Harbormaster to release the Drydock leases it has for hosts.

Test Plan: Tested it with a build plan that passes and fails; tested it with lots of builds running in parallel.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T5836

Differential Revision: https://secure.phabricator.com/D10208