add timestamps to crocodoc_documents

Change-Id: Ib1226b0aed4be89456b5c8f2ca250318ec254b13
Reviewed-on: https://gerrit.instructure.com/33317
Reviewed-by: Simon Williams <simon@instructure.com>
QA-Review: Simon Williams <simon@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Cameron Matheson <cameron@instructure.com>