replace emojione with emoji-toolkit

[rpetti]

This PR updates EmojiOne to the new package that replaced it, emoji-toolkit, which includes more current emojis. This fixes many broken emojis that will never be fixed in EmojiOne due to its deprecation.

I noticed that the original emoji handler code was replacing EmojiOne's CDN with a local static copy. It wasn't clear to me where to get the same for emoji-toolkit, and I don't think committing 200+MB worth of PNGs to git would be the best idea. I've left it using their CDN for now.