Add message reactions to state

[elliotwms]

Context

I'm working on a bot which interacts heavily with message interactions. It would be useful in both the application and in its tests if the state contained the message reactions on the message struct

## Proposal

Add message reactions to the State's message storage

This could be achieved by

• Adding state listeners for MessageReaction* events
• Adding TrackMessageReactions as config
• Using the event to determine the message
• Updating the State based on the event

Considerations

• This may be seen as overloading the Message struct, however Reactions []*MessageReactions already exists on the message
• Performance considerations, this includes a new class of event which may have an impact on larger bots. Should we set TrackMessageReactions to false by default?