Refactored course_nav middleware to isolate the django-wiki hack,
remove unnecessary code, and clarify what it's doing.
Merge pull request #2775 from edx/dcs/wiki-no-middleware

Refactored course_nav middleware to isolate the django-wiki hack, remove unnecessary code, and clarify what it's doing.
HD control is hidden initially. On first play of video, when
player.getAvailableQualityLevels returns a non-empty array, it will eventually display, if HD qualities are available (highres, hd1080, hd720). [BLD-937]
Added acceptance tests