Implement core_privacy for assignment plugins.

Testing can be completed by running the attached testwithmanager.php script in the Moodle root folder. Make sure you've purged the caches (php admin/cli/purge_caches.php), before running testwithmanager.php. Check that assignment plugins are compliant Check that all assignment plugins are null providers Check that the null reason is valid You should receive a script output like: Testing the compliant plugins:   ......   == assignment_offline (assignment_offline\privacy\provider) == Claims not to store any data with reason: 'The Offline assignment plugin does not store any personal data.' == assignment_online (assignment_online\privacy\provider) == Claims not to store any data with reason: 'The Online assignment plugin does not store any personal data.' == assignment_upload (assignment_upload\privacy\provider) == Claims not to store any data with reason: 'The Upload assignment plugin does not store any personal data.' == assignment_uploadsingle (assignment_uploadsingle\privacy\provider) == Claims not to store any data with reason: 'The Upload single assignment plugin does not store any personal data.'