AoC: Change leaderboard invite DM to ephemeral message slash command/button

[janine9vn]

Reliant on #19 being merged first.

I'd like to make the leaderboard invite code a slash command or semi-permanent button with an ephemeral message. This will allow people to keep their DMs closed and overall make the process smoother.

[D0rs4n]

I'll do it.