Feature request: direct links to individual leaderboards

[aoeixsz4]

Currently, there seems to be no obvious way to link to a particular leaderboard. I always have to go to https://tnnt.org/leaderboards and then select from the drop-down menu. I think it'd be nice if we could link to individual categories directly by a URL.

I'm thinking we probably want the hash fragment of the URL to be automatically updated whenever one changes board, so it's always clear that copying the URL at the top will bring you to where you currently are. Helpful for link sharing.

I have created a PR for this: #27