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