DEV: Remove JoyPixels emoji option (#12197)

- removes the option from site settings
- deletes the site setting on existing sites that have it
- marks posts using emojis as requiring a rebake

Note that the actual image files are not removed here, the plan is to
remove them in a few weeks/months (when presumably the rebaking of old
posts has been completed).

config/locales/client.en.yml

@@ -1819,7 +1819,6 @@ en:
       apple_international: "Apple/International"
       google: "Google"
       twitter: "Twitter"
-      emoji_one: "JoyPixels (formerly EmojiOne)"
       win10: "Win10"
       google_classic: "Google Classic"
       facebook_messenger: "Facebook Messenger"