Allow mod_assign to restore from mod_assignment backup files and completely remove mod_assignment

Before this patch: Enable both mod_assign and mod_assignment Create lots of instance of mod_assignment (all the different types). Add some data (submission/grades) to the assignment (the more the better) Create a backup of the course with all the assignments Install this patch Upgrade your site Verify that the admin received a notice about assignments that need upgrading Verify that accessing any of the assignments gives an upgrade required message Copy the urls of the existing assignments Verify that you can upgrade the assignments using the upgrade tool Verify you can access the assignments at their previous urls Verify that the grade book and calendar link to the correct assignment instance Restore the backup file into a new course Access the course and verify that all assignments are visible and have been upgraded to mod_assign as part of the restore Verify that you cannot create new instances of the old assignment module Verify that you cannot do anything with the old assignments but upgrade, backup or delete them (or indent/outdent).