DOC-252, add section on interpreting answer distribution
Merge pull request #3309 from edx/ahodges/documentation/DOC252

DOC-252, add section on interpreting answer distribution
Fix course urls in enrollment emails (LMS-2217)
InstDash: check Auto-Enroll and Email Students boxes by default
Merge pull request #3300 from edx/sarina/lms-2217

Fix course urls in enrollment emails (LMS-2217)
Update acceptance tests to use real duration of YouTube video.
Merge pull request #3318 from edx/valera/l62a

Update acceptance tests to use real duration of YouTube video.
Serve video HTML5 sources locally for acceptance tests