conditional to decide what content to post back
Filter most HTML from peer grading feedback

* Introduction ICE means we need its inline tags to get passed through
  to the feedback target, but we don't want malicious peer feedback
  providers introducing <script>, etc, so we use lxml.html.clean to
  scrub peer grading input.
* Adds feedback URL autoheating.
* Allows ICE <insert> and <delete> tags but not others.
Merge branch 'giulio/ice-stage' of github.com:edx/edx-platform into giulio/ice-stage