Introduce ManiphestTaskSearchEngine plus ManiphestTaskListControllerPro

Summary:
Ref T603. Ref T2625. Cutting this over is tricky because of Maniphest's existing saved queries. Plan here is:

• Build out the "pro" controller at /maniphest/query/.
• Once it's at parity, migrate custom queries.
• Nuke the old UI.

This provides a minimal implementation with no filter support.

Test Plan: Looked at /maniphest/query/, saw results technically available.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T603, T2625

Differential Revision: https://secure.phabricator.com/D6933