make global nav trays links read as buttons
make global nav trays links read as buttons

fixes ADMIN-2553

test plan:
- make sure help links are enabled in the rails console:
  - Setting.set('show_feedback_link', 'true')
  - restart your server
- the global nav items that open trays should read as buttons in
  screenreaders instead of links:
  - account, admin, courses, groups, and help
  - you'll have to be an admin to get the admin link, and a student in a
    group to get the groups link

Change-Id: I35cec18cd3451d526077706f331e0ced29ed71b9
Reviewed-on: https://gerrit.instructure.com/188077
Tested-by: Jenkins
Reviewed-by: Ed Schiebel <0ca8a83847a5889cf05cb219c0ab09f6b646243b@instructure.com>
QA-Review: Ed Schiebel <0ca8a83847a5889cf05cb219c0ab09f6b646243b@instructure.com>
Product-Review: Daniel Sasaki <ae19fabf7fe19dff310008c9b6a4d2ec84d311d8@instructure.com>

instructure/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,instructure/canvas-lms