Support emojis in flair text

[pkmnbumba]

Fixes #646

[Raia]

Copying from last PR:

Outstanding bugs:

• [x] The flair text preview window doesn't show previews for SVEx flair
• [ ] If someone's flair has just been updated and the user refreshes the page, the preview window doesn't properly show the new flair, but the sprite in the top right is updated. This can lead to a minor issue where if a user tries to set new flair text while their preview is showing an emoji with less characters than their current one, the client-side validation/preview won't be accurate. They still get the "Flairs too long" message from the server upon trying to submit though, which is good.

I don't consider either of the above to be blockers for deploying just so that we can reopen the sub, but we should get them fixed eventually.