A teacher can prevent or allow assigment submission changes

1. Log in as a teacher and create a new assignment, setting online text to yes and file submissions to no.
2. Log in as a student, click the 'Add submission' button, enter some text, then click the 'Save changes' button.
3. Click the 'Edit my submission' button, enter some more text, then click the 'Save changes' button.
4. Log in as the teacher, click the 'View/grade all submissions' link and in the edit column for the student's submission, select 'Prevent submission changes'.
5. Check that the status column now states 'Submission changes not allowed'.
6. Log in as the student and check that there is no longer an 'Edit my submission' button and that the submission status states 'This assignment is not accepting submissions'.
7. Log in as the teacher, click the 'View/grade all submissions' link and in the edit column for the student's submission, select 'Allow submission changes'.
8. Log in as the student and check that there is once again an 'Edit my submission' button.