R1:e5f867e0df34

Add Nuance daemons and item types

Summary:
Ref T10537. This adds an update daemon for pulling item data (e.g., figuring out who the author of a GitHub comment is) and routing items (e.g., sending them to a queue or applying them directly to a task).

Also adds `bin/nuance update --item X` for doing this manually for debugging.

And adds item types, for specializing item behavior. Previously, sources completely dictated item behavior, but I think we want something a little more flexible.

Test Plan…