provide high-res, color-coded favicons

fixes CNVS-13411

test plan:
  * open Canvas on a low-density display
  * verify that your tab icon looks like the old favicon
  * open Canvas on a high-density display
  * verify that your tab icon looks snazzy

  * verify that your tab icon is color-coded for the environment
  * run this in your browser console to try different environments:

require(['compiled/behaviors/favicon'], function(favicon) {favicon('production');})

  * verify that the color changes appropriately

Change-Id: I3c53eaf34d1abaa89f623382cbcbd92a3f2205e6
Reviewed-on: https://gerrit.instructure.com/35804
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Drew Bowman <dbowman@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Product-Review: Braden Anderson <braden@instructure.com>