Show some global statistics in viewer

[lucaro]

The old server would show some statistics, including the total number of submissions, the number of unique videos or the number of yet to be judged submissions in the viewer, see https://videobrowsershowdown.org/wp-content/gallery/vbs/IMG_0270-1.png
Would we want to have some statistics we could show as well?

[ppanopticon]

To add to this ticket: @lucaro and I had a conversation via Signal regarding how to best implement the limiting of submission previews:

• Currently, we derive the number of correct / incorrect / undetermined submissions from raw submission info, which is aggregated on the client side.
• The question is, if we'd rather have a dedicated statistics endpoint for such information. This would bring the advantage, that we can implement the limitSubmissionPreviews property in the backend (SubmissionInfoHandler) rather than in the frontend, which might reduce load.