Implement clear history for the API model

[rwood-97]

Code here:

reginald/reginald/slack_bot/bot.py

Line 300 in 93ec979

elif req.type == "slash_commands":

We need to update this in the ApiBot to make a call to the api to clear history on the VM versus self.model (which doesn't exist in the api bot).

[rchan26]

Might be able to close #88 if we finish this