Implications of "Modifying Submissions" on assignments after a due date.

If a student uploads an assignment it logs the time they submitted the work.

If the assignment activity is permitting late submissions, and a student that has already submitted something goes in to edit the submission after the due date and clicks "Save changes" to leave the page the submission time modified is changed, making it late, when all they intended to do may have been to review the files.

It's not obvious to the student what the implications of the "Save Changes" button is.

If it were possible to check if there were actually any changes to the submission and then record a change to the time modified (submission) value then this would eliminate this.

To reproduce:

1. Create an assignment, that allows late submissions and has a due date
2. Upload (as a student) a submission
3. Let the due deadline lapse
4. Edit the student submission (e.g. open the file picker and just click Save changes, making no changes)
5. In Grading screens Time modified is updated to the last date & time but the submission hasn't actually been changed.