fix "skip to content" overlapping main nav upon trigger
fix "skip to content" overlapping main nav upon trigger

Fixes: CNVS-29330

test plan:
- as a user with new ui
- refresh page load and immediately hit tab to trigger the
  keyboard "skip to content" link within the left-hand nav
- the blue "skip to content" link should now push the nav
  down vs overlapping it (this also works if user has
  high contrast selected)
- example of new behavior http://screencast.com/t/jj7uSVCRG

Change-Id: I2b697f7afcb7e4970e2fe60b7089a15b25359082
Reviewed-on: https://gerrit.instructure.com/79724
Product-Review: Colleen Palmer <387ba1943871e60965e6b44638a2f2e1ab9b8469@instructure.com>
Reviewed-by: Colleen Palmer <387ba1943871e60965e6b44638a2f2e1ab9b8469@instructure.com>
Tested-by: Jenkins
QA-Review: Myller de Araujo <e5c7c5e1b7f2ce5e9870cc98ec52b20b3006cb48@instructure.com>

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