Implement explicit build step ordering in Harbormaster

Summary: This implements support for explicitly marking the sequence of build steps.  Users can now drag and re-order build steps in plans, and artifact dependencies are re-calculated so that if you move "Run Command" before "Lease Host", the "Run Command" step has it's artifact setting cleared and thus the step becomes invalid.

Test Plan: Re-ordered build steps and observed dependencies being correctly recalculated.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Maniphest Tasks: T1049

Differential Revision: https://secure.phabricator.com/D7715