Deletable more cautious about ContentGroup removal

* ContentGroup was always being referenced, even for Deletables without
  any .contentgroup_set attribute. This threw an exception when you
  deleted one of these objects (such as Announcements).  Now we'll be more
  defensive.
  (fixes #1749)
* Some cleanups to comments
Merge pull request #1750 from Stanford-Online/jrbl-1749-delete-announcement-throws-execption

Deletable more cautious about ContentGroup removal