created new styling for CNVS-15493

new colors are based on $canvas-neutral

contrast for context on quizzes tab. can be viewed at '/quizzes'

rewrote quiz list text and icons to have single darken function. also, one less selector!

regular: #808080 on #fff :: 3.95:1
high contrast: #6d6d6d on #fff :: 5.17:1

Test plan:
- Navigated to quizzes tab. If there are no quizzes create one. 
- Verify contrast of publish icon, and 'pts' and 'question' text has been updated. You can verify the color contrast using http://webaim.org/resources/contrastchecker/
- After verifying enable high contrast.
- Repeat previous tests to verify high contrast color ratios.
 
Change-Id: I83ec5baf03d0264efcf0d7110c7f2be240f15b83
Reviewed-on: https://gerrit.instructure.com/41679
Reviewed-by: Chris Hart <chart@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Anna Koalenz <akoalenz@instructure.com>
Product-Review: Matt Fairbourn <mfairbourn@instructure.com>