Front page thumbnail viewer should not show 'superceded' entries

I just superceded `ETERNA_R00_0000` but somehow that triggered it's thumbnail to show up. The front page thumbnail viewer ('Latest Entries') should look at the `superceded_by` field and _not_ show superceded entries.

<img width="1172" alt="image" src="https://github.com/DasLab/Server_RMDB/assets/1672331/724b11a5-907b-4806-a1ef-6f98dfbc3440">
