Post-Event leaderboard tracking

At the end of the event keep the final leaderboard state where we can store it and display it, where in situation of KHI continuing for the foreseeable future the public can look over at some event results page and see the leaderboard across events.

PicoCTF does this [here](https://learn.cylabacademy.org/events)

No files, tests, or entry points are named. Compare the requested multi-event results page with the linked PicoCTF example, then locate how the current leaderboard state and events are represented; done means final leaderboards persist after an event and are publicly viewable across events.

Written by the indexing model from the issue text.