UI: vendorize github.com/elazarl/go-bindata-assetfs which is needed by ui-assets.go

vendor/github.com/elazarl/go-bindata-assetfs/doc.go

@@ -0,0 +1,13 @@
+// assetfs allows packages to serve static content embedded
+// with the go-bindata tool with the standard net/http package.
+//
+// See https://github.com/jteeuwen/go-bindata for more information
+// about embedding binary data with go-bindata.
+//
+// Usage example, after running
+//    $ go-bindata data/...
+// use:
+//     http.Handle("/",
+//        http.FileServer(
+//        &assetfs.AssetFS{Asset: Asset, AssetDir: AssetDir, Prefix: "data"}))
+package assetfs