avoid deadlocks creating submissions

on postgres < 3, lock users before referencing them to
avoid a deadlock

Change-Id: I4dc13c3a0b4d925fa30cbdad4447c4d62d3f75f3
Reviewed-on: https://gerrit.instructure.com/37390
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Ben Chobot <bench@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>