reduce size of all_site_admin_account_users cache key

store the bare minimum of ruby objects, instead of the full AR objects

on a large site admin test object, this cache went from ~64K to ~9K

Change-Id: I102127f9344dc801be9875b7e26092857061c1f4
Reviewed-on: https://gerrit.instructure.com/40182
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jacob Fugal <jacob@instructure.com>
Reviewed-by: Nick Cloward <ncloward@instructure.com>
QA-Review: Jeremy Putnam <jeremyp@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>