feature flag state-change callbacks

to enable for your feature, in your feature definition, use
  after_state_change_proc: ->(context, old_state, new_state) { ... }

test plan:
 - specs pass
 - a future commit will make use of this feature
   and can be tested directly

fixes CNVS-10357

Change-Id: I5510545802fd181707bc217e179ba7102eec9d37
Reviewed-on: https://gerrit.instructure.com/28570
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Bracken Mosbacker <bracken@instructure.com>
Product-Review: Bracken Mosbacker <bracken@instructure.com>
QA-Review: Bracken Mosbacker <bracken@instructure.com>