Add support for threads to `!slowmode` command

[minalike]

As it says on the tin.

Currently moderators+ are able to use !slowmode to get and set the slowmode for a given channel.

Let's add support for threads as well!

ETA: The command works when invoked within a thread, but not from outside that thread.

[ichard26]

Approved by Chris -> https://discord.com/channels/267624335836053506/965701967752097852/965708304963747932 (staff only)