Partially update Maniphest for handleProcess

Summary: Starts conversion of Maniphest to handleProcess, chopping up to reduce errors.

Test Plan: New Task, Edit Task, Change Priority, Move on workboard, view reports, batch edit tasks

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T8628

Differential Revision: https://secure.phabricator.com/D13773