Enables SR to understand the grayed out links in the navigation
Enables SR to understand the grayed out links in the navigation

fixes CNVS-12837

Test Plan:
  - Log in to canvas
  - Make sure you have a couple of nav links with no content
    for instance (no modules, no assignments, no something...)
  - They should show up as gray normally
  - Use a screenreader and there should be "* No content has been added"
    on each of the grayed out ones.

Change-Id: If6d51462cc123b53e14856b848d03de4a60daa12
Reviewed-on: https://gerrit.instructure.com/46608
Tested-by: Jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
Reviewed-by: Dan Minkevitch <2591e5f46f28d303f9dc027d475a5c60d8dea17a@instructure.com>
QA-Review: Clare Strong <40d6bb3ed7fef516c73274131d5a837c16de74fd@instructure.com>
Product-Review: Clay Diffrient <9dff2e5c98626d20d2848250c411e8009465bb87@instructure.com>

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