Jrbl contentgroup parents of self

@dcadams or @sefk please.

* Corrects overzealous exception thrown when ContentGroup parents were
      being added with add_child mistakenly.
* ContentGroup handling for File objects refined to have safer handling
      around missing and self-parent cases. UI now disallows selecting
      self-parenting, and corrects self-parented refs that pass through it.
* (closes #2480)