Add show vote slash command

[JohnnyJayJay]

The "show votes" button is better than having to copy some ID and pasting it in a command to view the votes, but unfortunately it stops working if the poll is too far up in the message history, because ephemeral messages get deleted instantly.

I see this more as something that Discord should fix/improve, as it's just bad UX. But if they don't do this anytime soon we could add a slash command as a fallback option, that would guarantee you to see the results, no matter how far in the past the poll is.