Reevaluate switching from PrimeReact to Mantle UI

The admin app uses PrimeReact. Its new major versions are no longer open source. We are still on the latest open-source release from PrimeTek (10.9.x), but that line will not move forward.

Mantle UI is a fork of the last open-source PrimeReact version that is supposed to be maintained. Check later whether it is alive and worth switching to, or whether another library fits better.