a way for accounts to opt-in to new styles and users to high-contrast

fixes CNVS-11426

The UI/UX team *really* wants to start doing some major style changes
to the canvas interface. Because it is a very visual change and especially
because we've let people hack the crap out of canvas styles with
their own css override file, any changes we do need to be behind a
feature flag so an institution can opt-in when they are ready.

This commit does some trickery so we actually create 4 different versions
of every stylesheet. one for each variant in:
legacy_normal_contrast legacy_high_contrast
new_styles_normal_contrast new_styles_high_contrast
and then sets the $use_new_styles and $use_high_contrast sass variables
accordingly in each.

now, in any sass file, you can do things like:
  @if $use_new_styles {
    background-color: red;
  } @else {
    background-color: blue;
  }

  @if not $use_high_contrast{
    font-size: 12px;
  }

test plan:

* in a production (minified assets) environment, ensure you see:
  <link href="/assets/common_legacy_normal_contrast.css... in the <head> of the page
* go to the account settings page for the domain_root_account you are on
* turn on the "Use New Styles" feature
* now verify that <link href="/assets/common_new_styles_normal_contrast.css...
  is in the <head>. There should not be any visible differences because we do
  not have any styles that target specifically $use_new_styles yet.
* now, go to /settings and turn on the user feature for "Use High Contrast Styles"
* verify that <link href="/assets/common_new_styles_high_contrast.css..
  is in the <head>
* again, turning that on will not actually change the way anything looks,
  the next commit (g/32863) will take care of that

Change-Id: I45ba4ddfe780c5c819fb995b61ebfc0a5325d418
Reviewed-on: https://gerrit.instructure.com/31881
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Ryan Shaw <ryan@instructure.com>
Product-Review: Ryan Shaw <ryan@instructure.com>
QA-Review: Ryan Shaw <ryan@instructure.com>
Reviewed-by: Bracken Mosbacker <bracken@instructure.com>