Allow users to specify names of build steps

Summary: Ref T1049. This provides a user-configurable name field on build steps, which allows users to uniquely identify their steps. The intention is that this field will be used in D9806 to better identify the dependencies (rather than showing an unhelpful PHID).

Test Plan: Set the name of some build steps, saw it appear in the correct places.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Maniphest Tasks: T1049

Differential Revision: https://secure.phabricator.com/D9816