Support Reaction Handling

[toolateforteddy]

There should be a parallel routing and message handling pipeline for emoji reactions. Both Slack and RocketChat fully support reactions.

Alternatively, we could expand our internal representation of an incoming message to also handle a reaction? One issue with this is that reactions are a much simpler message type, and yet also support both ReactionAdded and ReactionRemoved Events. As such, I think we would be better served by just having a different router object entirely.

Maybe orthogonal to this effort, we should make it much easier to attach a reaction to message as part of a response to a message. I am finding that in popular channels, having to respond inline, or even in a thread, is much more disruptive than being able to simply add a reaction.