R1:79d153b85d52

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…