Portfolio API code in the assignment module expected that the current
user is the author of the submission. Therefore the "Export to
portfolio" button did not work when the submission was viewed by a
teacher (eg at the page with the list of all submissions in the Advanced
upload assignment).

This patch introduces a new callback argument 'submissionid' that holds
explicit ID of the submission the export deals with. With it available,
we do not need to expect the current user is the author of the
submission.

The patch also cleans some strings used for portfolio callback
exceptions.