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>

lib/challenge/proofofwork/proofofwork.go

@@ -13,6 +13,7 @@ import (
 	"github.com/TecharoHQ/anubis/lib/localization"
 	"github.com/TecharoHQ/anubis/web"
 	"github.com/a-h/templ"
+	"github.com/gorilla/mux"
 )
 
 func init() {
@@ -24,7 +25,7 @@ type Impl struct {
 	Algorithm string
 }
 
-func (i *Impl) Setup(mux *http.ServeMux) {
+func (i *Impl) Setup(r *mux.Router) {
 	/* no implementation required */
 }