Add a missing DESC to newest_tip().

BEFORE: I forgot to tell SQLAlchemy to sort the results in descending
order, so newest_tip() *actually* returned the oldest tip.

AFTER: Now the newest_tip() function actually works, because we specify
that we wanted to sort in descending order.
Leverage the add_all method on the SQLAlchemy session.

BEFORE: I was using map and the add method.

AFTER: We still have to use the map idiom for deletes (when relevant),
but we use the more concise add_all method instead of the map idiom for
adding multiple Tips to the session.