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)
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