SAK-27920 Allow feedback to be assigned to multiple people at once.

This allows feedback to be given to multiple assignment submissions at the same time when marking an assigbment.
Thanks to Juan for the patch

git-svn-id: https://source.sakaiproject.org/svn/assignment/trunk@314145 66ffb92e-73f9-0310-93c1-f5514f145a0a