[a11y] Hides course menu from keyboard only users
[a11y] Hides course menu from keyboard only users

Makes it so that the hidden course menu gets removed from
the tab order while it is hidden.  It is then added back
when it is shown.

fixes CNVS-22770

Test Plan:
  - Enable new navigation
  - Load up a course with the viewport set small (< 1200px)
  - The hamburger menu should be shown
  - Tab through and you should go from the end of the help menu through
    the breadcrumbs and to your content.
  - Expand the viewport to > 1200px
  - You should now be able to tab through to the course menu
  - Reduce the viewport again
  - Tab to the hamburger menu and activate it.
  - The course menu should now be in the tab order
  - Verify that with screenreaders the hamburger menu is never "shown"
    to them and that all the course menu items are always "visible"
    to screenreaders

Change-Id: I0f7d25ad29e6896f662396528b206c8e34473457
Reviewed-on: https://gerrit.instructure.com/62141
Tested-by: Jenkins
Reviewed-by: Dan Minkevitch <2591e5f46f28d303f9dc027d475a5c60d8dea17a@instructure.com>
QA-Review: Nathan Rogowski <2e8aa918660411855c6d44d5bb2da677aa033255@instructure.com>
Product-Review: Clay Diffrient <9dff2e5c98626d20d2848250c411e8009465bb87@instructure.com>

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