download token: make a token page

- token page helps communicating a single link to multiple assets
- provide an extra layer in case of "miss click"
- ui: just link the page, not every asset of each download token.

cmd/dkl-local-server/ws.go

@@ -37,6 +37,7 @@ func registerWS(rest *restful.Container) {
 			Route(ws.POST("/store.tar").To(wsStoreUpload).
 				Consumes(mime.TAR).
 				Doc("Upload an existing store")).
+			Route(ws.GET("/downloads/{token}/").To(wsDownloadPage)).
 			Route(ws.GET("/downloads/{token}/{asset}").To(wsDownload).
 				Param(ws.PathParameter("token", "the download token")).
 				Param(ws.PathParameter("asset", "the requested asset")).