Implementing NYT's ICE track changes to Open Ended Assessements

[jbau]

@VikParuchuri This is a modified and reworked version of https://github.com/edx/edx-platform/pull/956 that addresses all your comments on that PR.

Still short a test, but will add that today. But please take a look at this if you happen upon it with or without the test

Thanks,
Jason

Squashed commits to facilitate cherry-picking. Committers and
original commit messages are:

• ==> @caesar2164, * ==> @jrbl, ~ ==> @jbau
• Added boolean to OE problem CMS settings to turn on ICE track changes
• Added ICE init and tracking start/stop to HTML
• Conditional in Peer Grading HTML for ICE container
• CSS styling for ICE container and insertion/deletion elements
• Added class to track changes HTML
• Added ice.min.js as well as included it in the js requirements for peer grading
• Use track_changes in peer_grading_problem
• Refactor peer_grading_problem to lookup track_changes variable by problem location.
• Whitelist ICE insert, delete tags
• Adds ICE multi-user-editor insert and delete tags to the peer grading sanitize_html whitelist.
• add reset button
• 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 and tags but not others.

~ add helptext re: EXPERIMENTAL FEATURES

~ address a bunch of @VikParuchuri review comments on ICE for OEE
~ remove extraneous code
~ change to new xmodule accessors
~ refactor filtering

~ fix broken test test_peer_grading_problem

~ Create track_changes.coffee and remove <script> from
peer_grading_problem.html
~ Remove all reliance on id in favor of local scoping and classes

[jbau]

@caesar2164 Note that I refactored the ICE stuff pretty extensively: I converted your <script> tag into a .coffee file.

[jbau]

I just made a backup copy of giulio/ice-pr at giulio/ice-pr-bak and force push my branch over it, so let's just use the old PR.