Dropbox is switching to short-lived access_token OAuth. This commit adapts Cloud::DropboxStorage to use refresh_token similarly to how other Storages do: by introducing a DropboxTokenRefresher.
It is used instead of plain CurlJsonRequest. It just checks if received JSON has access_token-related error and tries to refresh it, then repeats the original request transparently for the caller. If no such errors found, it just passes the info to the caller.
