fix: enable double slashes for optional path variables

Closes #754

This implementation is flawed, it's making the pass-challenge route
return 404s. I'm not entirely sure why this is happening, but I'm
pushing this for now in case there's some low hanging fruit.

Signed-off-by: Xe Iaso <me@xeiaso.net>

xess/xess.go

@@ -10,6 +10,7 @@ import (
 
 	"github.com/TecharoHQ/anubis"
 	"github.com/TecharoHQ/anubis/internal"
+	"github.com/gorilla/mux"
 )
 
 var (
@@ -20,8 +21,6 @@ var (
 )
 
 func init() {
-	Mount(http.DefaultServeMux)
-
 	//goland:noinspection GoBoolExpressions
 	if anubis.Version != "devel" {
 		URL = filepath.Join(filepath.Dir(URL), "xess.min.css")
@@ -31,8 +30,10 @@ func init() {
 }
 
 // Mount registers the xess static file handlers on the given mux
-func Mount(mux *http.ServeMux) {
+func Mount(r *mux.Router) {
 	prefix := anubis.BasePrefix + "/.within.website/x/xess/"
 
-	mux.Handle(prefix, internal.UnchangingCache(http.StripPrefix(prefix, http.FileServerFS(Static))))
+	r.PathPrefix(prefix).
+		Handler(internal.UnchangingCache(http.StripPrefix(prefix, http.FileServerFS(Static)))).
+		Name("xess")
 }