188 bad date range handling

Improving how changes to certain item fields affect the derived fields of its ancestor objects. In particular, items now store their from- and to- dates separately from the (derived) `date_from` and `date_to` fields of the `object` table.

The code and methodology is cleaner now, too.

This change involves some structural DB changes. Applying this change effectively also involves running a one-time script on the DB. I'll be in touch with you separately about that, Deborah.