Migrate the bot from discord.py async to discord.py rewrite

[lemonsaurus]

In order to make use of features like the AutoShardedBot and because rewrite is just a lot easier to organize, this bot should be migrated over to discord.py's rewrite branch.

Migration instructions can be found here: https://discordpy.readthedocs.io/en/rewrite/migrating.html

This shouldn't be a big job, considering how little discord.py code the current bot has.