MDL-31936 Add missing file level docblock to course/reset_form.php

Spotted while working on the workshop reset feature. Assigning copyleft holder
to Petr as the original author of the file.
MDL-31936 Fix workshop_assessment::get_overall_feedback_content() processing

This is unrelated to the issue but was spotted while testing it. The order of
format_text() and file_rewrite_pluginfile_urls() was incorrect and threw
debugging message. Fixed now.
MDL-31936 Delete files attached to workshop submission or assessment on removal

The methods workshop::delete_submission() and workshop::delete_assessment() did
not delete files (embedded and attachments) associated with the given
submission or assessment. This is fixed now.

Additionally, the delete_assessment() method now cleans-up records from the
table workshop_grades, too. This internal workshop API still does not give
workshop subplugins a chance to clean up their data, should they store them in
their own tables instead of the workshop_grades one. This should be improved in
the future yet.
MDL-31936 Add more options to the workshop reset form

This is built upon original work by Michael Hughes. We now have separate
settings for deleting workshop submissions, assessments and for resetting the
workshop phase.
MDL-31936 Make workshop generator able to create submissions and assessments

This is a basic implementation of the actual content generation in the workshop
generator. Note the we do not generate actual grading form data (which is what
the grading strategy plugins would do), just their results.
MDL-31936 Improve unit tests for the workshop reset feature