Add very basic routing to Nuance

Summary:
Ref T8783. Sort out some relationships and fields:

• Make Items 1:1 with Queues: each item is always in exactly one queue. Minor discussion on T8783. I think this is easier to understand and reason about (and implement!) and can't come up with any real cases where it isn't powerful enough.
• Remove "QueueItem", which allowed items to be in multiple queues at once.
• Remove "dateNuanced", which is equivalent to "dateCreated" in all cases.

Then add really basic routing:

• Add "Default Queue" for Sources. New items from the source route into that queue.
• (Some day there will be routing rules, but for now the rule is "always route into the default queue".)
• Show queue on items.
• Show more / more useful edit history and transactions in several UIs.

Test Plan:

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T8783

Differential Revision: https://secure.phabricator.com/D13988