Convert 'high-contrast' from user pref to feature flag

fixes: CNVS-11426

a few months ago we started trying to enable users to turn
on some "high contrast" css changes. The way we did that was
with a override css file that would go and override existing
style rules to set them to new values.  the main difference
here is that it is done at the *source of where the variable
is declared* instead of *after the fact, overriding what was there*
this will make things a lot easier to maintain.  for example, see
the part where instead of having a huge selector to override a bunch
of things that look like links to be darker, we just go and set the
$linkColor sass variable to be a little bit darker.

test plan:
* before you check out this code, go to /settings and turn on the
  'high_contrast' user theme.
* run the db migration in this commit
* now go to /settings and make sure that the 'high contrast' feature
  is enabled.
* verify that with that feature enabled, when you go to /styleguide
  you should see higher contrast buttons, link text, and backgrounds
* it wont be exactly the same as when you had the 'high contrast' theme
  enabled because some of the high contrast things weren't super important
  (specifically, the darker grey borders on some elements). but the most
  important changes, making the contrast between text and the background
  around it, should be there.

this essentially reverts:
"adds high-contrast theme user preference"
b232dcf44804b49c380a733adacdb50db5636e58.

this also makes jmadsen's change to make the green
alert bar a little darker (to comply with contrast)
in g/31881 apply across the board to all of our
green things so they are all compliant. (in addition,
now if they have high contrast enabled, it will be
even darker green)

Change-Id: Iee7d81bb5160e86f1900dbf4a249364b1ddfa7f1
Reviewed-on: https://gerrit.instructure.com/32863
Reviewed-by: Cody Cutrer <cody@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Ryan Florence <ryanf@instructure.com>
QA-Review: Nathan Rogowski <nathan@instructure.com>
Product-Review: Ryan Shaw <ryan@instructure.com>