More accessible focus states for buttons, links, tabs

Fixes: CNVS-14477 CNVS-14478 CNVS-14479 CNVS-14886

All link elements in Canvas should now be marked with
a clear light blue outline when focused by keyboard
navigation. The same visual treatment has also been
applied to buttons and tabs.

Screencast: http://screencast.com/t/QxzcHXsC8s

Test plan:

- Use the keyboard to tab through the navigations
  and various pages of Canvas.
- Make sure the focus state looks the same as in
  the screencast.
- Note: keyboard navigation is implemented differently
  in every browser. Make sure your browser preferences
  are set to allow keyboard navigation.

Change-Id: I9702d2381fc05126106988c3b5523cced9a8f6d4
Reviewed-on: https://gerrit.instructure.com/41091
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Colleen Palmer <colleen@instructure.com>
QA-Review: Nathan Rogowski <nathan@instructure.com>
Product-Review: Chris Hart <chart@instructure.com>
Product-Review: Colleen Palmer <colleen@instructure.com>