MDL-35726: Use a different form identifier for each form when using "Save and next"

MDL-35726: Use a different form identifier for each form when using "Save and next"

This prevents a bug where the next student feedback will default to the values that
were on the last submitted form. This is a similar case to having multiple instances
of the same form on the same page - each one needs a unique identifier to distinguish
between them. In this case I include the $rownum which changes as you go to each student
in the list.