Adds new colors and updates variables stylesheet

fixes CNVS-14507

This introduces the new color variables for Canvas.

Changes:
* renamed _variables.sass to _variables.scss
* moved _variables from /themes/ to /base/
* combined _canvas_colors into _variables
* removed _canvas_colors.scss (since it's now a part of _variables)

Test plan:

Nothing should look different in Canvas.

Change-Id: I1a94961c253a00b2a9c9710c726af036464198e0
Reviewed-on: https://gerrit.instructure.com/39113
Reviewed-by: Chris Hart <chart@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Matt Fairbourn <mfairbourn@instructure.com>
Product-Review: Colleen Palmer <colleen@instructure.com>