log auditor events to the rails log

closes CNVS-14174

test plan: Generate some audit events, such as login/logout and grade
changes. grep your canvas log for AUDITOR lines, which should contain
a json object with the attributes of the audit event.

Change-Id: Icbf67b94120e2e22ba656e673edc04ab30de5945
Reviewed-on: https://gerrit.instructure.com/37572
Reviewed-by: Jacob Fugal <jacob@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Jeremy Putnam <jeremyp@instructure.com>
Product-Review: Brian Palmer <brianp@instructure.com>