Also fix:
  - double-serialization of keys
  - using course keys instead of ids
  - using ids instead of course keys
  - empty course keys returning empty string instead of None
Other consequences:
  - If a model object is created using a string for an OpaqueKeyField,
    the field will stay the string, even after a save(). When the object
    is pulled back from the DB, the field will then be an Opaque Key
    object.
  - If a model is created without providing an opaque key for the
    OpaqueKeyField, the model instance returns an empty string
    for the key - it used to return None.