create ensure-contrast sass func for easier text color picking

whenever you are setting the color of text that will be
on top of some background, you should use this new
`ensure-contrast` function.

example usage:
.simple-example {
  // will output '#eee' in normal contrast, but will output '#686868' for people that
  // $use_high_contrast so that it meets the 4.5:1 contrast requirement
  color: ensure-contrast(#eee, #fff)
}

// more thorough example showing how you should be doing it with variables:
$page-bg-color: white;
$primary-brand: blue;
$link-color: ensure-contrast($primary-brand, $page-bg-color);
a { color: $link-color }

this also goes through and updates some of the places
where we were doing explicit overrides for high-contrast
to use this new function, which also provides examples
of how to use it

test plan:

* nothing should change in normal contrast
* with high contrast turned on, the text of these
  things should be at least a 4.5:1 contrast ratio.
* one particular thing to check is the placeholder on
  input elements (fixes: CNVS-13725)

closes: #CNVS-14022

Change-Id: Ic16ab84c2dfac1d600ea705abf134522762b398e
Reviewed-on: https://gerrit.instructure.com/37375
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jon Willesen <jonw@instructure.com>
Reviewed-by: Sterling Cobb <sterling@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: Ryan Shaw <ryan@instructure.com>