Slightly simplify SearchAttach controller

Summary:
I want clean this up enough that I can land D595 without making a complete mess,
here's a small simplification.

Move object load logic into PhabricatorObjectHandleData.

Test Plan: Attached tasks and revisions, merged tasks.
Reviewed By: aran
Reviewers: tuomaspelkonen, jungejason, aran
CC: aran
Differential Revision: 724