R1:cd651001b6c6

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