R1:403073c989b4

Provide a workflow to restart Harbormaster builds

Summary:
Ref T10867 for original use case. This workflow provides a plausible way for administrators to stop the daemons when performing upgrades or maintenance, then bring those daemons back up without resulting in the failure of builds that were running at the time.

On our organization's phab install, builds are running 24/7. The majority of these builds last for at least several minutes, and contain build steps which fail if interrupted and then…