change draft state to 'allowed' instead of 'hidden'

test plan:
  * as an account admin (but not site admin), verify
    that you can see the draft state feature in your
    account;
  * as a teacher, verify that you cannot see the draft
    state feature in your courses until the account admin
    has set the feature to 'allow.'

Change-Id: I5d2f54d9ee0b50742b8011b1d9b777c9825030a1
Reviewed-on: https://gerrit.instructure.com/32255
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Clare Strong <clare@instructure.com>