quizdown for MC and SA
Merge branch 'master' of https://github.com/Stanford-Online/class2go into jbau-quizdown
Quizdown with video assoc
small tweaks
mathjax rendering for previewing explanations
migration + code for saving import quizdown
actually add migration
remove unneeded import
moved remove() call for aElem after all references
some examples/support for surveys -- resp w/o ans
better slugs if things are small
Merge pull request #2132 from Stanford-Online/jbau-quizdown

Jbau quizdown