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
