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.

Ran unit tests.
Tested to make sure Review UIs for common file types (images,

  markdown) still worked correctly.