Allow Nuance item types to provide actions for items

Summary:
Ref T10537. This allows item types to expose item actions. Eventually these actions might be things like "promote to task", "tweet reply", "ban user forever", etc.

For now, provide a simple action which shows a raw item in a dialog.

Test Plan:

Reviewers: chad

Reviewed By: chad

Subscribers: Luke081515.2

Maniphest Tasks: T10537

Differential Revision: https://secure.phabricator.com/D15504