Revert "Revert "create PermissionsSerializer for serializing permissions""

This reverts commit e340d1a76283126b583fcc031bf44849efc6e906.

Change-Id: I4b9fa5beed4bbcf8441c5f3a79a0e03df42163d4
Reviewed-on: https://gerrit.instructure.com/29185
Reviewed-by: Jason Madsen <jmadsen@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Stanley Stuart <stanley@instructure.com>