This patch adds unit tests for RB.UploadAttachmentView. The recent refactoring 

of the view led to some regressions that were not caught until the changes were 

already landed. With unit tests, we can help reduce the likelihood of this 

happening in the future.

New tests:

- Cancel button has the correct state when the dialog is initialized

- Cancel button closes the dialog when clicked

- Upload button has the correct state when the dialog is initialized

- Upload button has the correct state when a file is uploaded and when it is 

removed

- Upload button sends request to create a file attachment when clicked

- the dialog is populated with the right information when updating a file 

attachment