Add an ItemCommand queue to Nuance

Summary:
Ref T10537. Generally, when users interact with Nuance items we'll dump a command into a queue and apply it in the background. This avoids race conditions with multiple users interacting with an item, which Nuance is more subject to than other applications because it has an import/external component.

The "sync" command doesn't actually do anything yet.

Test Plan:

Reviewers: chad

Reviewed By: chad

Subscribers: Luke081515.2

Maniphest Tasks: T10537

Differential Revision: https://secure.phabricator.com/D15506