Separate the commands into their own files

[mklarz]

We should do some fast clean-up, as we currently have 3 categories of commands. bot.py contains our core commands, ctftime.py contains the commands for ctftime, whilst ctfs.py contains the commands for both the main CTF channel, but also the challenge channels.

I suggest we separate the commands into two files, ctf.py for the commands that are used in the main channel, and challenges.py for the commands used inside a challenge channel.

This would give us a better overview, and since we don't use the bot for anything else, this shouldn't be an issue, and this is also considered best-practice. We could also look into moving the methods within these files into their own file(s), for example utils.py.