Inch toward using ApplicationSearch to power related objects

Summary:
Ref T4788. Fixes T9232. This moves the "search for stuff to attach to this object" flow away from hard-coding and legacy constants and toward something more modular and flexible.

It also adds an "Edit Commits..." action to Maniphest, resolving T9232. The behavior of the search for commits isn't great right now, but it will improve once these use real ApplicationSearch.

Test Plan: Edited a tasks' related commits, mocks, tasks, etc.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4788, T9232

Differential Revision: https://secure.phabricator.com/D16189