Drive modular task relationships through a new "relationships" controller

Summary: Ref T11179. This is basically a "pro" controller to replace the SearchAttach controller. It does basically the same stuff, just in a (mostly) more modern and modular way.

Test Plan:

• Added and removed mocks.
• Added and removed revisions.
• Everything worked just like it did before.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11179

Differential Revision: https://secure.phabricator.com/D16163