MDL-47397 editor_atto: Properly close confirm dialogue in Behat test
MDL-47397 assignfeedback_editpdf: Copy stamps to other students in team

During a team submission the stamps were attached to the user that
was graded and not to the other users. This could lead to errors when
generating the PDF for each user.
MDL-47397 assignfeedback_editpdf: Tests use reliable way to annotate PDF

The drag and drop step did not work, so we moved to using the stamps
to annotate the PDF. They do not rely on drag&drop and produce the same
effects.