add reset_content action to courses api

test plan:
* should be able to post to the
 "reset_content" action (see the Courses API
 documentation) to reset the course content
 (equivalent to the UI action)

closes #CNVS-17670

Change-Id: I6c8e96dcf6cc21ac06d599197338f6764c629c56
Reviewed-on: https://gerrit.instructure.com/46312
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>