don't destroy notification policies in Facebook preferences

find the existing policy if there is one, and reset missing ones instead
of destroying them

Change-Id: Iec50b29c5977ac727af1225f6c80260a5ef09d3d
Reviewed-on: https://gerrit.instructure.com/29998
Reviewed-by: Rob Orton <rob@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>