Use tabs on build targets and allow build steps to have a description

Summary:
Ref T1049. This uses tabs on build targets to hide the configuration details and variables by default, instead promoting the target name, it's status and a description of the build step. The description is a new field on each build step.

The primary advantage of having a description on build steps is that DevOps can configure appropriate description information (including any troubleshooting information for build failures) on build steps, and developers who have builds fail against their code review can then look at this information.

Test Plan: Viewed a build plan and saw the appropriate information.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T1049

Differential Revision: https://secure.phabricator.com/D10093