Reaction objects with custom Emoji are partial. If we know of this Emoji
(can find it on this client) then inject it. Otherwise, leave it as a
hollow Emoji. We can still react with a hollow Emoji, but can't get other
metadata about it.
Reactions can be be standard emojis, or custom server emojis.
Adds
- add/remove_reaction
- get_reaction_users
- Messages have new field reactions
- new events - message_reaction_add, message_reaction_remove
- new permission - add_reactions
