cbc629f4f4
Defining custom DownloadCommandLink types for the changes screen is cumbersome and defining them as default types makes them easily available for other uses, for example: to potentially download branches, the project dashboards, or even the meta config branch. It also keeps some of the special wiring between the DownloadCommandLink and some of the other download classes such as the DownloadCommandPanel internal to the download package. Change-Id: Ia29f82c78d65d1311e09e0cdedc1e0ce78e7c759