Only The Latest Backup/Submission Should Be Autograded

[epai]

From experience, it seems like the auto promotion process of backups during auto grading is a little buggy:

• The last backup is flagged for submission, even if that backup is past the deadline
• If multiple submissions are sent, the latest one is graded, even if it is past the deadline

In both of these cases, only the latest on time backup and submission should be graded.

[epai]

Note: this is my initial assessment -- I will double check the source code to make sure this is the case

[knrafto]

Possibly related to #947?