Add a contextual "scope" dropdown for searches
Summary: Add a "Search for ... in (document group)" thing that picks the current
scope based on the current application.
Test Plan: Conducted searches in several browsers.
Reviewers: btrahan, skrul
Reviewed By: btrahan
CC: aran, epriestley
Maniphest Tasks: T858
Differential Revision: https://secure.phabricator.com/D1610