invalidate cached stream item instances after item deletion

test plan:
* with caching enabled, create an announcement or other
 object that creates items in the "Recent Activity"
* delete the item
* should refresh the activity right away and not
 show the now deleted item

closes #CNVS-962

Change-Id: I28b2165c238ad074c345db23b64384439d8b8efb
Reviewed-on: https://gerrit.instructure.com/44477
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Dan Minkevitch <dan@instructure.com>
QA-Review: Jahnavi Yetukuri <jyetukuri@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>