be more defensive of special account creation

fixes CNVS-16632

don't auto-create unless the setting was previously nil, and we're
*not* in production mode. production mode *requires* using
rake db:initial_setup.

test plan:
 * existing canvas install should work
 * a fresh canvas install should also work

Change-Id: Iee4b0ead053aef220aac5f54c519174ce1b41843
Reviewed-on: https://gerrit.instructure.com/43845
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jacob Fugal <jacob@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: August Thornton <august@instructure.com>