Accessibility: Added navigation landmark to the the top bar.

Fixes CNVS-12503

Test plan:
- Using a screen reader, visit any page in Canvas.
- Verify that the top bar has a navigation landmark with a label of
  "User Menu"

Change-Id: I0ba93c519427d83f32e74f99ede2d2501bd76172
Reviewed-on: https://gerrit.instructure.com/35510
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cameron Sutter <csutter@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Product-Review: Aaron Cannon <acannon@instructure.com>