Speed up leaderboard filtering with Alpine

This commit replaces the hand-written search functionality with one based on rendering the leaderboard through an Alpine.js template, similar to how we do it for the event filter.

A difference between this implementation and the event one is that here we do not have a new API handler, but instead embed the required data in the page using Django's json_script approach, which I was not aware of when we implemented events.

The speedup is quite impressive, at the cost of a slight increase in required code. We do not add new dependencies though.