extract color gem

test plan:
 * colors on the old calendar still display correctly
 * click assignments top nav button
   * sidebar colors should be displayed correctly

fixes CNVS-11423

Change-Id: I07677569d121797ba52382e5e29be7b16878031e
Reviewed-on: https://gerrit.instructure.com/30700
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
Product-Review: Mark Severson <markse@instructure.com>
QA-Review: Mark Severson <markse@instructure.com>