Color Variables for Canvas UI

fixes CNVS-15117

These introduce color variables for Canvas! With these
fancy variables we can now start getting color consistency through
Canvas! Huzzah!

UI Testing:

- Compile sass
- Compile styleguide (bundle exec rake css:styleguide)
- There are two separate styleshees: one for legacy, one for new;
these are being hidden or shown based on css
- See QA procedure below for expected testing results

QA Testing:

- With the new_styles tag off, in /styleguide you should see a
"Canvas Colors - Legacy" section with Base Colors & High Contrast
- Width new_styles flag on, in /stylguide you should see a
"Canvas Colors - New" section with Base Colors & High Contrast
- You should never see two Canvas Color sections at a time

Change-Id: I2536ce5bf49bc0de2166cdcd8f3be15be9ff6625
Reviewed-on: https://gerrit.instructure.com/40309
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Matt Fairbourn <mfairbourn@instructure.com>
Product-Review: Colleen Palmer <colleen@instructure.com>
Reviewed-by: Chris Hart <chart@instructure.com>