create k12 specific stylesheets

closes: #CNVS-10113

this is the start of creating a new k12 specific
look-and-feel.  To start using this, turn on the
'k12' feature flag for an account.

test plan:
this should do nothing for existing users
but to test that this is working you can
do:
Account.find(<id of your account>).enable_feature!(:k12)
and then make sure that the stylesheets being loaded
on the page are coming from either:
/stylesheets_compiled/k12_normal_contrast/
or
/stylesheets_compiled/k12_high_contrast/
(depending on if you have turned on high contrast
for yourself)
if everything looks just like it did before, this works.

Change-Id: I9355d1498ab4f6587faf137b129a998044a3bbeb
Reviewed-on: https://gerrit.instructure.com/37926
Reviewed-by: Colleen Palmer <colleen@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Chris Hart <chart@instructure.com>
Product-Review: Colleen Palmer <colleen@instructure.com>