Handle autoban filtering in DMs (#1914)

An autoban trigger being sent in DMs caused the ban to fail, but for it to still be registered in the database. That is becuase the ban command uses the `ctx.guild.ban` method, but in DMs `ctx.guild` is None. This commit solves it by overriding the `context.guild` field.

bot/exts/filters/filtering.py

@@ -370,7 +370,8 @@ async def _filter_message(self, msg: Message, delta: Optional[int] = None) -> No
                             # This sends the ban confirmation directly under watchlist trigger embed, to inform
                             # mods that the user was auto-banned for the message.
                             context = await self.bot.get_context(msg)
-                            context.author = self.bot.get_guild(Guild.id).get_member(self.bot.user.id)
+                            context.guild = self.bot.get_guild(Guild.id)
+                            context.author = context.guild.get_member(self.bot.user.id)
                             context.channel = self.bot.get_channel(Channels.mod_alerts)
                             context.command = self.bot.get_command("tempban")