Enhance Maniphest custom queries
Summary:
Improve the custom query interface:
- Allow search for tasks not in projects.
- Allow search for tasks with no projects.
- Allow custom search to include author/owner constraints.
Test Plan: Searched for various sorts of tasks.
Reviewers: btrahan
Reviewed By: btrahan
CC: aran, epriestley
Maniphest Tasks: T911
Differential Revision: https://secure.phabricator.com/D1722