messageable user: exclude bookmark from serialization (rails2)

fixes CNVS-13309

test plan:
  * set up pg_collkey on rails 2
  * open user settings page
  * verify that the page loads

Change-Id: I4b9bac1b48a974d0f415b9db7fa67468eddea237
Reviewed-on: https://gerrit.instructure.com/35780
Reviewed-by: Drew Bowman <dbowman@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Braden Anderson <banderson@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>