React UI: Fix "<path-prefix>/new" -> "<path-prefix>/new/" redirect (#6319)
React UI: Fix "<path-prefix>/new" -> "<path-prefix>/new/" redirect (#6319)

Fixes https://github.com/prometheus/prometheus/issues/6163

Signed-off-by: Julius Volz <d7748b1e6f41fd74500c5ccae0980eab88c2ef61@gmail.com>
kangwoo/prometheus,kangwoo/prometheus,alileza/prometheus,prometheus/prometheus,alileza/prometheus,keegancsmith/prometheus,alileza/prometheus,alileza/prometheus,prometheus/prometheus,GoogleCloudPlatform/prometheus,Gouthamve/prometheus,kangwoo/prometheus,Gouthamve/prometheus,GoogleCloudPlatform/prometheus,prometheus/prometheus,GoogleCloudPlatform/prometheus,Gouthamve/prometheus,keegancsmith/prometheus,GoogleCloudPlatform/prometheus,GoogleCloudPlatform/prometheus,prometheus/prometheus,alileza/prometheus,kangwoo/prometheus,keegancsmith/prometheus,keegancsmith/prometheus,prometheus/prometheus,keegancsmith/prometheus,Gouthamve/prometheus