Make the mobile menu available in "/mail/"

Summary: Ref T13244. See https://discourse.phabricator-community.org/t/left-hand-menu-not-responsive-on-mobile/2358.

Test Plan:

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13244

Differential Revision: https://secure.phabricator.com/D20093