Creating new CTFs and challenge channels do not check the channel limit on the guild

[mklarz]

The bot does not check if the channel limit has been reached for discord guild. The bot adds a channel to its DB and attempts to create the channel, but then errors out.

We need to add a check on !create and !add.

Might be worth adding a check for !voice as there is a limit there as well #29

[mklarz]

There is also a limit of 50 channels per category, which will cause issues if someone is playing multiple CTFs or a larger CTF, as the working and done category will most likely reach the limit.

This means that we need to be able to extend the "done" category into multiple ones. This can be done easily once #12 has been completed.

[mklarz]

Regarding the last comment, the logic behind archive solves this issue by appending a counter to the end of the category name, and working + done should do the same. See #72