Rename methods

Renamed the following `Assignment` methods, as per the IRC chat with @reidka and @david-yz-liu:
- `#past_due_date?` to `#past_all_due_dates?`
- `#what_past_due_date` to `#section_names_past_due_date`
- `#section_past_due_date?` to `#grouping_past_due_date?`