Added an asynchronous each to the ExtensionHookModel using jQuery Deferred Objects.

The addition of the new method allows client code to attach callback functions for

cases when the entire collection was successfully processed or cases when one or 

more of the items in the collection failed to be processed.
This uses jQuery Deferred Objects so it should be compatible with non ES6 versions

of reviewboard/non-ES6 browsers.

Ensured that it worked with the new code added in the 'blocked' review requests to the right.

Exiting each still works.