Fixes #1650

Uses the correct exam.image relationships in Video model commit() revert() is_synched()
Changes Video edit action to properly handle exams (using exam and not exam_id in a few places)
Code that fixes #1644

All of the following changes eliminate the javascript console errors
* Re-enables the "video question" thumbnails
* Kelvinator highlighting transitions are now cued by the manifest time,
  instead of polling
* Made the cueing a separate function called in the deferred chain.
#1635 in-video exams/surveys just resume video
trying seeked event instead of seeking in video player
Migrations for #1513 -> make all grading fields floats
Merge pull request #1654 from Stanford-Online/jbau-jan2-fixes

Jbau jan2 fixes