Filtered decks should shuffle identical values

[dae]

All our standard deck orders end with with equivalent of , random(), so that cards due on the same day don't always appear in the same order for example. We don't do this for filtered decks, which can cause things like reviewing ahead to show cards in a predictable order. Perhaps we should be doing something similar to the standard review process instead, updating order_and_limit_for_search() to sort by fnvhash(id, mod) like the standard review process does.