Collapse "Show unassigned tasks" in Maniphest into a parameterized owners typeahead
Summary: Ref T4100. Support viewer(), members(), and add a new none().
- 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.
Reviewed By: btrahan
Maniphest Tasks: T4100
Differential Revision: https://secure.phabricator.com/D12470