Smooth out some UI/UX issues in Harbormaster

Summary:
Ref T8096. Fixes a few bugs and glitches.

• Set build completion time when handling a message.
• Format duration information in a more human-readable way.
• Use a table for build variables.
• Fix up container PHIDs on diffs (a touch hacky, should be OK for now though).

Test Plan: Browsed around the UI.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T8096

Differential Revision: https://secure.phabricator.com/D13382