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