Add "maniphest.search" Conduit API endpoint

Summary: Ref T9964. This is a basic implementation of the new "maniphest.search" endpoint.

Test Plan: Clicked the button in the web UI, got meaningful results back.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9964

Differential Revision: https://secure.phabricator.com/D14760