Restore population of `ownerOrdering` to ManiphestTasks

Summary:
Ref T4110. This denormalized field used to power "Group By: Assigned" got dropped in the T2217 migration at some point.

Restore its population, and fix all the data in the database.

Test Plan: Ran migration, verified database came out reasonable-looking. Reassigned a task, verified database. Ran a "Group By: assigned" query.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4110

Differential Revision: https://secure.phabricator.com/D7602