…ncy based

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