Use the new ExtensionFinder to find static files in extensions.

This just adds the new ExtensionFinder in Djblets to the list of static
file finders, so that extension-bundled static/ directories will be
searched.
Add a hook for working with comments in the review dialog.

This new hook, ReviewDialogCommentHook, allows an extension to display
data or provide UI for a comment in the review dialog. It takes a view
type that will later be instantiated and passed the comment as a model.
The view can do whatever it needs to with the model, including providing
UI to set new data on it.