Toggle TrackChanges with platform-wide setting

* Use self.runtime.platform_settings['ORA2_TRACKCHANGES_URL'] to find
  location of change tracking javascript library.
* TrackChanges is fully disabled if this variable is unset.
* TrackChanges may be enabled on a per-problem basis using a convenient
  checkbox on the Studio settings panel for the peer assessment.
* Documentation updates
* Bleach changes as they are marshaled for display, eliminating unsafe
  markup.