Expose "parent task" and "subtask" relationships to "edge.search"

Summary: Ref T12337. This just fills out a couple more task relationships.

Test Plan: Viewed the edges in the Conduit console, queried for them.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12337

Differential Revision: https://secure.phabricator.com/D17465