automatically run tinyMCE.triggerSave() on form.submit()

test plan:
 - when quiz time expires, textarea questions should autosave

Change-Id: I1a0638471ffae9412a407c2dba9333e254ca472e
Reviewed-on: https://gerrit.instructure.com/31843
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Derek DeVries <ddevries@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Product-Review: Aaron Shafovaloff <ashafovaloff@instructure.com>