pandapub: integrate with wiki page updates

test plan:
 * Set up and configure PandaPub. (See commit with change id
   Ifddcbd335293c2a29f532b1e5fd44c23c8b910c4 for instructions.)
 * Open a wiki page in a course in one window.
 * In another window, open the same page, and edit it, and save it.
 * You should right away see the warning pop up in the original
   window that the page was edited while viewing.

Change-Id: I5afcbd34f79385d6466cbbc7c9fc9c211126cf0d
Reviewed-on: https://gerrit.instructure.com/40477
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Sean Lewis <slewis@instructure.com>
Reviewed-by: Jacob Fugal <jacob@instructure.com>
Product-Review: Zach Wily <zach@instructure.com>