Move artifacts and build target messages into tabs

Summary: This moves artifacts and build target messages into tabs.

Test Plan: Viewed build plan, saw the tabs appear when the steps had appropriate artifacts and / or messages.

Reviewers: #blessed_reviewers, epriestley, chad

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D10161