Implementing NYT's ICE track changes to Open Ended Assessements

@VikParuchuri, could you give this PR a look?

- 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
- (@jrbl) Refactor peer_grading_problem to lookup track_changes variable by problem location.
- (@jrbl) Whitelist ICE insert, delete tags
- (@jrbl) Adds ICE multi-user-editor insert and delete tags to the peer grading sanitize_html whitelist.
- add reset button