grab_next_visits: avoid time interval calculations in PostgreSQL

When the database is in a non-UTC timezone with DST, and a `timestamptz

• interval` calculation crosses a DST change, the result of the

calculation can be one hour off from the expected value:

PostgreSQL will vary the timestamp by the amount of days in the
interval, and will keep the same (local) time, which will be offset by
an hour because of the DST change.

Doing the datetime +- timedelta calculations in Python instead of
PostgreSQL avoids this caveat altogether.