Allow Nuance item types to provide actions for items
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.
Reviewed By: chad
Maniphest Tasks: T10537
Differential Revision: https://secure.phabricator.com/D15504