MDL-27550 workshop: display the feedback for the submission author when the workshop...

MDL-27550 workshop: display the feedback for the submission author when the workshop is closed

The method user_picture::fields() was not available when these SQL
statements were originally written. Now instead of hard-coding the list
of returned fields, the user_picture is asked for the list.

Together with this change, methods workshop::get_submission_by_id()
and workshop::get_submission_by_author() now return the information
about the user who provided the feedback and overwrote the grade.