Added ungraded problems/content pages.

Fixes DOC-863, also adds info from edx-doc preface

@mhoeber these two copies of the preface got way out of line sometime
around the portal release. I’ve made them identical as well as adding
the docs@edx.org sentence at the beginning for DOC-863.

Set versioning of cldr-data.

Display percentage of correct submissions in table.

Updated django announcement branch to fix compatibility issue.

All performance pages point to "performance/Performance_Answers.html"

Fixes 2 sloppy phrases

Re-architected Course Validation Scripts

Fix syntax and typo in README.md

@stroilova please review

Ahodges/doc/answer dist

Update performance page to catch and throw 503.

 - Storing data in Elasticsearch instead of writing to CSV
 - Using a base reporter class to allow for the addition of future report generators
 - Added report generator for graded content data
 - Added reporter generator for graded content page screenshots

Revert "Fix syntax and typo in README.md"

Fix syntax and typo in README.md

Darkened performance nav text.

Error page will be displayed for 502s from course API.

Updated performance navigation to highlight next action.

Hide table when no submissions are available in assignments page.

Stacked bars will show all labels unless space is limited.

Ignoring Empty Assignment Types in Grading Policy

These non-truthy assignment types are not useful for our purposes, so they will be filtered out and ignored.