Custom submission times not set when Extension is created after backups

[epai]

Looks like the custom_submission_time attribute is not set when the extension was created after backups are made. Not sure what the best fix is - could either retroactively change the submission times of previous backups when an extension is made, or check to see if any Extensions were made in the Backup.submission_time property, and then set it if it is. Leaning more towards the first, though, as the second would be much more inefficient (multiple database queries necessary).

If opting for the first method, we'd also need to reset the submission times if the extension is ever modified/deleted.

[epai]

Alternatively, we could just set the custom_submission_time right at the api-layer before we return the backup info, which is what I think I'll do so the attribute is set retroactively.

[stanfieldchu]

I like the initial proposal. Is there an easy way to do it at run time of autograding?