use bright calendar colors for low-contrast users

test plan:
  * disable high-contrast stylesheet
  * load calendar
  * add events for multiple contexts
  * verify that the colors used are bright

  * enable high-contrast stylesheet
  * load calendar
  * verify that the colors used are dull

Change-Id: Id8b39aa1c683994411e9111e14667aab1f157fd1
Reviewed-on: https://gerrit.instructure.com/41072
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Ryan Shaw <ryan@instructure.com>
Reviewed-by: Matthew Wheeler <mwheeler@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Product-Review: Braden Anderson <braden@instructure.com>