Refactor speed control.
Add fixes.
Update docstrings.
Removed unused helpers.
Upgrade JSDraw.
Return 404 instead of 500 for missing forum thread

Also show a more specific error message in the front end. This change
only has an effect if using cs_comments_service commit 1d71330 or later.
Merge pull request #3307 from edx/gprice/deleted-thread-404

Return 404 instead of 500 for missing forum thread
in Studio, render compact placeholders for inline discussions.

JIRA: FOR-429
Merge pull request #3295 from edx/jsa/disc-preview

in Studio, render compact placeholders for inline discussions.
Support for Duplicate and Delete of XBlock Container Leaves (disabled under Feature Flag).
fix for fuzzy prompt on container page
Merge pull request #3234 from edx/nimisha/container-duplicate-delete

Nimisha/container duplicate delete
Merge pull request #3231 from edx/anton/refactor-speed-control

Video: Refactor speed control.
Merge pull request #3308 from edx/adam/hotfix-js-draw

Upgrade JSDraw.
fix for fuzzy prompt on Studio Pages
Merge pull request #3324 from edx/frances/fix/studio-pages-prompt

Fix for fuzzy prompt on Studio Pages
Merge pull request #3322 from edx/release

Release
Serve video HTML5 sources locally for acceptance tests
More changes to enable local video sources in acceptance tests