add visual indicator to RCE color selector

fixes CNVS-6705
This commit adds a visual highlighting of the color chosen when selecting
colors in the tinymce editor.

Test Plan
1. Go to a page that includes the RCE (the quiz edit page includes one)
2. Switch between background and font colors using the color selector.
- There should be a visual indication of which color you chose (a
  shading of the background of the selected color)

Change-Id: Id3af0b7d36ee8f8fb3b069ef4fa9906558e9da2b
Reviewed-on: https://gerrit.instructure.com/28380
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Myller de Araujo <myller@instructure.com>
Reviewed-by: Derek DeVries <ddevries@instructure.com>
Product-Review: Josh Simpson <jsimpson@instructure.com>