UI Changes for static page parent setting

* UI changes necessary to be able to set the ContentGroup for a static
  page from its edit screen, rather than only at page creation time.
* This has bearing on correction of #1559.
* This branch should be revisited when #1458 is addressed.
* Todo: plumbing to edit ContentGroup entries when changes are made
Back-end support for editing static page parents

* (closes #1482)
* Also back-end support for #1458, but javascript item filtering still
  present.
* Known issues: moving a parent to be a child of another parent makes
  the moved parent's children invisible.
ContentGroup convenience methods
ContentGroup.add_child does corner cases bettter

* More careful handling of case where parents get reassigned to be
  children of other parents.
* Give up Try/except in favor of using a data structure and if/else
  (should be faster)
* (closes #1482)
AdditionalPages javascript updates

* Ban on other types being ContentGroup parents to AdditionalPages
  partially lifted.
  (closes: #1458)
Docstring fix for ContentGroupManager.getByFieldnameAndId
Factor out ContentGroup manipulations

* Pull ContentGroup manipulations out of the various create and edit
  dialogs and put them into a callable function.
  (fixes #1603)
Child static pages display in list style

* Child static pages default to list-style display
* Non-file child items in button-style display have their title
  attribute as button text, rather than their item type.
  (fixes #1604)
Bugfix to exam editing parent/child assignment

* In refactoring parent/child assignment code, exam editing code was
  broken. This fixes.
#1530 - move scoring to the correct tables.

Removes appropriate:
ExamRecordScore
ExamRecordScoreField
ExamRecordScoreFieldChoice

Creates new:
ExamRecordScore
ExamRecordScoreField

Updates score in:
ExamRecord
ExamScore

Also, I replaced the colons in the export file names with dashes as
colons are not good in Windows file names.
Merge pull request #1587 from Stanford-Online/jrbl-1482-parent-assignment-static-page-edits

Jrbl 1482 parent assignment static page edits
Merge branch 'master' of http://github.com/Stanford-Online/class2go into
dca_new_working2

Conflicts:
	main/courses/exams/views.py
Grade export files contained a blank line at the end that was causing an
warning on import. Removed the blank line if file was going to be larger
than zero size. The blank line is still needed for otherwise zero length
files as S3 throws an error.
fixes #1728 adds ordinal numbering
Fix for #1713

Hide explanations when transitioning to new questions, redisplay when
Check Answer clicked

	modified:   templates/exams/videotest.js
	modified:   templates/exams/view_exam.html
Merge pull request #1735 from Stanford-Online/wescott-master-1713-fix

Fix for #1713
Merge pull request #1734 from Stanford-Online/jbau-ordinal-numbering

fixes #1728 adds ordinal numbering
Merge pull request #1733 from Stanford-Online/dca_new_working2

Move CSV scoring to the correct tables #1530
Merge branch 'master' of github.com:Stanford-Online/class2go into sef-1729-interactive-exercises