Adjusting indexes to make favorites query more efficient, based on feedback from...

Adjusting indexes to make favorites query more efficient, based on feedback from ops.

fave_user_id_idx index changed from (user_id) to (user_id,modified), so the timestamp ordering can be done straight from the index while we're looking up the user's notices.
Added to 08to09.sql and 08to09_pg.sql; may need to be run manually by folks doing development.
(No harm if you don't update it, the favorites tab/rss feed will just stay inefficent.)