[harbormaster/abort-builds] Support aborting builds in Harbormaster

Summary: Ref T1049. This adds support for aborting builds in Harbormaster.

Test Plan: Tested it in production.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: yelirekim, traviscline, joshuaspence, Korvin, epriestley

Projects: #harbormaster

Maniphest Tasks: T1049

Differential Revision: https://secure.phabricator.com/D11870