Add a new `newest_classification_at` timestamp field to UPP

[marten]

This new field should be the timestamp that gets touched on each classification, instead of the updated_at.

We're touching updated_at for each classification so that the front-page can sort UPPs by updated_at and sort projects based on when you've last contributed. With the updated_at name however, it's not obvious why we go through the trouble of writing to it for every classification.