Show command transactions in Harbormaster builds

Summary:
Create transaction, editor, etc, and move command generation over to editor.
Show in a timeline in the buildable page.

Also prevent Engine from creating an empty transaction when build starts (Fixes T4885).

Fixes T4886.

Test Plan: Restart builds and buildables, look at timeline.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T4885, T4886

Differential Revision: https://secure.phabricator.com/D9110