Bug: Webhook check on guild de-integration fails

util.discord.check_webhook() expects CablesContext, but we are passing it self, and attempting to read self.bot. This doesn't work, as CablesBot is initialized in self._bot.

Change that func input to bot: CablesBot and pass self._bot, as we've done elsewhere.

While you're at it, move this de-integration/nuke checking logic to its own module, as we shouldn't have complex logic like this in events.dispatch