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:

• This still does nothing.
• Ran bin/nuance update --item 15.
• Saw an item route to a default queue.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10537

Differential Revision: https://secure.phabricator.com/D15441