Auto-extension rule

[Lourenzutti]

Currently, the auto-extension flow checks if the registration date is higher than the assignment[unlock_at], if it is, then, the student gets config.latereg_extension_days days of extension.

Instead, we can consider checking whether reg_date + extension_days > due_date. If it is, we give the student extension, if it isn't, we don't as the student will have at least extension_days days to do the assignment.