In order to make Review UI classes more flexible, we're going to consolidate all the functionality down into the base class. We currently had the `ReviewUI` class which has an opaque `obj` member, and the `FileAttachmentReviewUI` class which assumes that `self.obj` is a `FileAttachment`. This change takes the functionality from `FileAttachmentReviewUI` and moves it into the base class, conditionalizing it based on the type of `self.obj`. This will allow us to add additional conditions for other types of objects, such as allowing it to be a `FileDiff`. This also adds a new flag to the ReviewUI classes for whether they support operating on a `FileAttachment`. This is used when selecting the best ReviewUI for a given object. Testing Done: - Ran unit tests. - Tested to make sure Review UIs for common file types (images, markdown) still worked correctly.