[perf] leaderboard loads and renders excessive amounts of data

[duanwilliam]

edit: oh i guess this is similar to #513?

Currently, visiting the leaderboard fetches all users in the given time range. For the 2021-2022 school year, this amounts to ~160kb of data, and for all time, ~500kb (which will only increase over time). The leaderboard page eager loads all these entries as well, resulting in a DOM tree of nearly 9000 nodes when viewing just the 2021-2022 leaderboard. This is further antagonized by the fact that each entry contains an image, which results in a large amount of data being transferred.

It could be beneficial to lazy load leaderboard entries, e.g. fetch and render the next 50/100/whatever entries when the user scrolls to a certain point before the end of the currently rendered leaderboard.
In the meantime, we can add loading="lazy" to the profile images to at least save a bit on that end.