Make HarbormasterBuild a better source of truth about restarting/resuming/stopping

Summary: Ref T1049. The logic in the BuildEngine is a little different from the logic on the Build itself. Make these more consistent, and make queued commands more private.

Test Plan: Restarted, stopped, and resumed a build.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1049

Differential Revision: https://secure.phabricator.com/D7897
Improve restart/stop/resume UI

Summary:
Ref T1049. Improves the UI:

  - Pending commands, like "stopping", are shown separately from the current status.
  - Pending commands are shown on the list view.
  - Builds can be restarted, stopped and resumed from the list view.
  - Add a missing crumb.

Test Plan:
{F99022}

{F99023}

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1049

Differential Revision: https://secure.phabricator.com/D7898