Collapse "Show unassigned tasks" in Maniphest into a parameterized owners typeahead

Summary: Ref T4100. Support viewer(), members(), and add a new none().

Test Plan:

• Used all new functions.
• Batch edited tasks with unassign action.
• Saved a query from master, upgrade it to this patch, checkbox migrated cleanly into a "no one" token.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4100

Differential Revision: https://secure.phabricator.com/D12470