fix "Skip to Content" link
fix "Skip to Content" link

test plan:
 - go to a Canvas page
 - tab until "Skip to Content" appears
 - press Enter
 - the content should be focused

fixes ADMIN-2657

Change-Id: Ia8db6e2bab40a16eb116f4b5c9b0ad9b630f40d1
Reviewed-on: https://gerrit.instructure.com/193339
Tested-by: Jenkins
Product-Review: Jeremy Stanley <b3f594e10a9edcf5413cf1190121d45078c62290@instructure.com>
Reviewed-by: Ryan Shaw <ea3cd978650417470535f3a4725b6b5042a6ab59@instructure.com>
QA-Review: Ryan Shaw <ea3cd978650417470535f3a4725b6b5042a6ab59@instructure.com>

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