Add a spoiler option to channel settings

[fiveseven808]

For certain reasons, I've edited lien 507 of stablecog.py to now read file = discord.File(fp=buffer, filename=f'SPOILER_{queue_object.seed}-{count}.png') which hides the result. It would be great to have this as a parameter while using the /draw command and also setting it as a channel default.

[Kilvoctu]

Added it to projects, but I'm not sure when I'll get around to it.