Add ability to clear chat history in slack bot

[rwood-97]

After the end of each 'conversation' with the chat bot, we should clear the history so that irrelevant history isn't kept unncessarily.
Two options for this are:

• User manually clears chat history though interaction on slack. eg.. could be to emoji a message or use one of the / commands
• We implement a timeout system where the chat history auto clears after xx amount of time. Could be something like after 3 hours or 24 hours.

Might be good to implement both of these.

[rchan26]

Do we know what happens if the chat history gets too long? Do older chat histories just get cut off, or does it crash due to not enough context length left?

Edit: see #101

[rwood-97]

#71 may also be relevant here as both are relating to emoji react events.