Update the list of known Discord emoji

[anoadragon453]

The repo we pulled our emojis from updated their list pretty significantly in July 2022: https://github.com/sevenc-nanashi/discord-emoji

Instead of copying the source of it to our repo, we should just pull in the discord-emoji library from PyPi (probably pin the version just in case) and fish out the table.DISCORD_TO_UNICODE dictionary.

Edit: Although we would need to implement an emoji blacklist or something in that case, as we've removed some emoji from the default list.