xblock lms integration for TIM

Basic/Static Application HTML

* basic app skeleton HTML
* response HTML
* peer evaluations
* self evaluation
* grade status
* UI messages

Conflicts:
	apps/openassessment/xblock/openassessmentblock.py

TIM-126: Peer criteria support explanations

* Rubric criteria options now support an optional <explain>
  child, which is used as explanatory text for that particular
  criterion option, e.g.:
  5 - Very Well (indicates intelligent use of persuasive
                 argument to, etc etc.)
* Sample explanations added to the Poverty sample rubric to
  show them in use.
* parse_xml now calls external rubric parser to make testing easier
* basic unit tests for xml parsing confirm things seem to work
* max available points calculated at rubric ingestion time now

Sanchez/xblock peer eval

In addition to the score API cleanup, there is doc, test fixes,
and some refactoring around API use.

Make Submission Admin page more useful by inlining student item and scores

* TIM-39 partial: peer evaluators being shown other students' submissions and
  a rubric, and feedback in the form of radio buttions is being solicited.
  This is *mostly* plumbed through to the API.
* TIM-53 complete
* TODO: Needs tests, needs rubric_criteria refactoring

Co-authored-by: Joe Blaylock <jrbl@jrbl.org>

Jrbl/xblock submit return type

* Removed commented-out nascent Rubric model implementation

* Basic parsing of XML rubric's in @ormsbee's proposed format
* Rubrics displayed to user after they've made at least one submission
* Multiple rubrics on the workbench with different evaluation criteria

Creating a debug page for evaluations.

* Updates to API error checking and reporting to make it possible to
  report error conditions back to the user (though in production we'll
  generally not want to burden them with this information.)
* CSS and javascript fixes to reflect same.
* Beginning of HTML escaping of prompts introduced, but that part is
  incomplete as yet.

WIP: Outlining the API intended for Peer Grading Workflow.

Added some additional documentation. Some quick cleanup.

Almost entirely documentation.

Add XBlock and xblock-sdk dependencies separately