Require access to Still via method

This commit adds a safeguard to accessing the still from a specific
component by requiring access via the component.GetStill method.

internal/dis/component/auth/next/next.go

@@ -58,7 +58,7 @@ func (next *Next) getInstance(r *http.Request) (wisski *wisski.WissKI, path stri
 	}
 
 	// find the slug
-	slug, ok := next.Config.HTTP.SlugFromHost(url.Host)
+	slug, ok := component.GetStill(next).Config.HTTP.SlugFromHost(url.Host)
 	if slug == "" || !ok {
 		return nil, "", httpx.ErrBadRequest
 	}