Filter out deleted tasks when building review queue

[andymatuschak]

Our data store is architected such that tasks are never deleted; they're just flagged as deleted. When building a review queue (e.g. in app's databaseManager and backend's notification bits), we should exclude deleted tasks. Doesn't actually affect anyone but me yet, but will once we add a UI for deleting.

(Unfortunately, this'll probably muck up our indexing / predicate strategies a bit—we can't just add the isDeleted column to our composite indexes without mucking up order when not filtering by isDeleted)