Implement interface for adding, editing and deleting build steps on plans.

Summary: This implements an interface for adding new build steps, editing existing build steps and deleting build steps from build plans. It uses the settings definitions on the build implementation to work out what fields should be displayed on the edit page.

Test Plan:
See screenshots:

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Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Maniphest Tasks: T1049

Differential Revision: https://secure.phabricator.com/D7500