Support PhabricatorSpacesInterface in ApplicationSearch UI

Summary: Ref T8441. Does what it says, provided other conditions (like using the new SearchField stuff) are fulfilled.

Test Plan:
{F473836}

{F473837}

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T8441

Differential Revision: https://secure.phabricator.com/D13171