http: improved directory listing with new template from Caddy project

This includes a new directory listing template which was originally
from the Caddy project (used with permission and copyright attribution).

This is used whenever we serve directory listings so `rclone serve
http`, `rclone serve webdav` and `rclone rcd --rc-serve`

This also modifies the tests so they work with the original template which
is easier to debug.

cmd/serve/http/http_test.go

@@ -26,11 +26,13 @@ var (
 
 const (
 	testBindAddress = "localhost:0"
+	testTemplate    = "testdata/golden/testindex.html"
 )
 
 func startServer(t *testing.T, f fs.Fs) {
 	opt := httplib.DefaultOpt
 	opt.ListenAddr = testBindAddress
+	opt.Template = testTemplate
 	httpServer = newServer(f, &opt)
 	assert.NoError(t, httpServer.Serve())
 	testURL = httpServer.Server.URL()