Summary:
Depends on D9806. This implements the build simulator, which is used to calculate the order of build steps in the plan editor. This includes a migration script to convert existing plans from sequential based to dependency based, and then drops the sequence column.
Because build plans are now dependency based, the grippable and re-order behaviour has been removed.
Test Plan: Tested the migration, saw the dependencies appear correctly.
Reviewers: epriestley, #blessed_reviewers
Reviewed By: epriestley, #blessed_reviewers
Subscribers: epriestley, Korvin
Differential Revision: https://secure.phabricator.com/D9847
