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
