fix contrast issues with authentication select menu
fix contrast issues with authentication select menu

Fixes: CNVS-33503

test plan:

- as an admin navigate to "Admin" in the main left navigation
- select your account/university name
- select "Authentication" from the subnav
- the "Choose an authentication service" select menu should now
  pass color contrast for both high-contrast and non-high-contrast
  users (side note: the hover state has been updated to match the
  brand primary on hover like found in the kyle menu)
- example: http://www.screencast.com/t/lqg1Y4sqH

notes: the react_select_box stylesheet has been updated to use
variables vs the hard-coded hex numbers that were there

Change-Id: Idbb698355767ca409d8f5705ea047cd25cad4a3f
Reviewed-on: https://gerrit.instructure.com/96165
Tested-by: Jenkins
Reviewed-by: Stephen Jensen <af2501a686d970a4ae53690c2b6f7e448f31f7df@instructure.com>
QA-Review: Dan Sasaki
Product-Review: Pam Hiett <7b533437add1ce4ea4ed79979fa88e0e484e554b@instructure.com>

grahamb/canvas-lms,SwinburneOnline/canvas-lms,matematikk-mooc/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,matematikk-mooc/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,djbender/canvas-lms,djbender/canvas-lms,SwinburneOnline/canvas-lms,venturehive/canvas-lms,grahamb/canvas-lms,venturehive/canvas-lms,grahamb/canvas-lms,instructure/canvas-lms,matematikk-mooc/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,venturehive/canvas-lms,matematikk-mooc/canvas-lms,grahamb/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,venturehive/canvas-lms,grahamb/canvas-lms,SwinburneOnline/canvas-lms,djbender/canvas-lms,SwinburneOnline/canvas-lms,sfu/canvas-lms,instructure/canvas-lms