Revert heavy blue bordered focus states

Fixes: CNVS-17099

This change scopes the "blue highlight boxes" to only
appear when Canvas is in high-contrast mode.

Test the following in both high- and regular-contrast
modes:

- Tab and click around Canvas, checking links, tabs,
  buttons, and toggle switches.
- Make sure you only see the blue highlight boxes
  (http://cl.ly/image/3F2S1i0a0A0l/highlight-box-example.png)
  in high-contrast mode.

NOTE: some browsers, like Chrome, will still render their
own focus-state styles on tab-focus, and that's ok.
(for example: http://cl.ly/image/1J0F2h3c1F15/chrome-focus-example.png)

Change-Id: Ib2b25376cc1611ae41bc99a735b95cd37a7fbb9e
Reviewed-on: https://gerrit.instructure.com/44882
QA-Review: Nathan Rogowski <nathan@instructure.com>
Reviewed-by: Colleen Palmer <colleen@instructure.com>
Product-Review: Colleen Palmer <colleen@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>