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.
Unstyle list of exams in child material listings

* Unstyle child exam list in the course materials items display
  (fixes #1651)
#1635 in-video exams/surveys just resume video
get_live_datetime handles missing ready mode item

* When course_materials.get_live_datetime gets a thing that has no ready
  mode, it helpfully returns a bit of HTML that can be used to tell the
  user this.
  (closes #1600)
trying seeked event instead of seeking in video player
ContentGroups handle deletion of target items

* Fixes display bug causing deleted child items to be displayed in live
  mode (fixes #1652)
* Fixes semantic bug causing item deletion to leave vestigial
  ContentGroup entries. New semantic introduced for parent/child
  deletion:
  - Deletion of the target object also deletes its ContentGroup entry
  - Furthermore, if the target object is a ContentGroup parent, its
    related ContentGroup children are promoted to be parents.
  (fixes #1481)
Merge pull request #1653 from Stanford-Online/jrbl-1600-error-handling

get_live_datetime handles missing ready mode item
Add exception case for ContentGroup entry delete

* Raise an exception when a deleted object has multiple ContentGroup
  entries.
Merge pull request #1656 from Stanford-Online/jrbl-1652-contentgroup-item-deletion-handling

ContentGroups handle deletion of target items
Merge pull request #1657 from Stanford-Online/jrbl-1651-child-item-list-style-appearance

Unstyle list of exams in child material listings
Merge branch 'master' of github.com:Stanford-Online/class2go into sef-1449-grader
Migrations for #1513 -> make all grading fields floats
Merge pull request #1654 from Stanford-Online/jbau-jan2-fixes

Jbau jan2 fixes
Assessment student data report (#1594).

Replaces old problemset student data report with report for new exam
types.
Addendum to #1594

Tweaked the Assessment Student Data report. If the human_name for the
field is empty show the field name instead.
fixes #1598 fixes #1599 use xml to assoc video->exam
tool to merge a group of problem set reports to end up w one row per student
put the username in quotes
Merge pull request #1663 from Stanford-Online/jinpa-master-survey-merge-tool

tool to merge a group of problem set reports to end up w one row per stu...