clear context's feature flag cache before API update

test plan:
 - in an instance with redis caching enabled,
  1. set draft state to "allowed" in a root account
  2. in a console, make the cache think the feature flag doesn't exist
     Rails.cache.write(account.feature_flag_cache_key('draft_state'), :nil)
  3. reload the account settings page
  4. you should be able to set draft state to "on" or "allowed"
     with no AJAX error, and the change should stick

fixes CNVS-13484

Change-Id: I4a9664858f3dfcbd3d2d2999a9e1e62b1231bceb
Reviewed-on: https://gerrit.instructure.com/36123
Reviewed-by: Cody Cutrer <cody@instructure.com>
QA-Review: Nathan Rogowski <nathan@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>