Confirmation dialog for orphaned files

As we explore the interface around file management in #1448, we might want to think about how the user interface should work in regards to orphaned files which will deleted once they are removed from the content to which they have initially been uploaded.

Right now if you remove a file from a node, it will be cleaned up by garbage collection if it is not being used on any other nodes. We might want to ensure that the user knows that will happen, by providing a confirmation message during the save that orphans the file, perhaps along with the options to either delete the file now or mark it permanent.