messageable user: exclude bookmark from serialization

fixes CNVS-13309

test plan:
  * open user settings page with in an environment with
      pg-collkey enabled
  * verify that the page loads

Change-Id: Ic3a8b969df6a8a6c6c0b5e3c0a2d740eb324c025
Reviewed-on: https://gerrit.instructure.com/35615
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Reviewed-by: Joel Hough <joel@instructure.com>
Product-Review: Braden Anderson <banderson@instructure.com>