messageable user: exclude bookmark from serialization (rails2)
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 <43bd25f9469d5056946537962fe98ec04e66c401@instructure.com>
Tested-by: Jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
Product-Review: Braden Anderson <a6cc2cef7d25f27847876bdb81651755fcebacf0@instructure.com>
QA-Review: Trevor deHaan <85df72112e8a9484fec7b3db2e31789c2c2d4e07@instructure.com>

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