Fix: Unable to assign or reassign content to a group once content item is created

[lyndsey]

I believe this is a group issue and not an opensocial issue as such. There is a user experience problem here though because content needs to be recreated if it's added to the wrong spot, or needs to be reassigned.

Have not checked drupal.org for corresponding task. Assigning to @johnatasjmo as per comment in task DrupalOpenLearning/drupaldojo-old#17