create "K12" feature flag

closes: CNVS-14347

Yay, we finally have a way to tell if someone is a K-12 
or higher ed account!

All new features that are K-12 specific should be behind
this feature flag.

test plan:

this should be hidden from accounts but you should
be able to turn it on manually by doing
Account.find(<id>).enable_feature! 'k12'

this feature flag doesn't actually turn anything
on yet, so no visual changes should happen.

Change-Id: I567e13928607acc941708f3505c31e3af47fda14
Reviewed-on: https://gerrit.instructure.com/37925
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Colleen Palmer <colleen@instructure.com>
Product-Review: Hilary Scharton <hilary@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>