Fix issue #1295: Check to see if every group being collected had only one TA assigned to them.

There is now an additional expression to be checked before displaying the collect button for a TA. It checks that every submission has just one TA, if that is false the button does not appear. 

I did some manual testing. I assigned two TAs to a set of submissions, and one TA to the rest of the submissions. The collect button does not appear for the two TAs that are assigned the same submissions, but does appear for the TA assigned to their own set of submissions.