discussions: fix commenting on topics containing videos

fixes CNVS-7974

test plan:
  * create a discussion topic containing this HTML:

<object width="480" height="385" data="http://www.youtube.com/v/y8mzDvpKzfY?fs=1&amp;hl=en_US" type="application/x-shockwave-flash"></object>

  * view the topic
  * verify that the video is displayed
  * verify that you can add a comment at the root level on the topic

Change-Id: I05b20f4635ca999044fe90c9334de2bde402be9c
Reviewed-on: https://gerrit.instructure.com/36221
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Drew Bowman <dbowman@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Product-Review: Braden Anderson <braden@instructure.com>