Allow users to grant themselves roles by reacting to messages

[KazWolfe]

One of the core features of this bot should be the ability to allow users to grant themselves certain roles by using message reactions.

Users should:

• Only be permitted to do this in a single (configurable) channel
• Only be granted roles upon reacting to a message with a specific emoji
• Lose the role after removing their reaction to the message

To keep things clean, users should be allowed to promote one of a few selected emojis to a message.

The config of this feature should be in the following format:

    "promotions": {
        "11223344": {
            "thinking": "22112211"
        }
    }

In the above example, reacting with :thinking: to message ID 11223344 will grant you role 22112211.