discord: fix /posts command inside DMs

Fix a bug where the /posts command failed when used in a DM channel,
because we were trying to fetch a `nsfw` key that didn't exist.

app/logical/discord_slash_command/post_embed.rb

@@ -61,7 +61,7 @@ class DiscordSlashCommand
     end
 
     def is_nsfw_channel?
-      command.channel.fetch("nsfw")
+      command.channel.fetch("nsfw", false)
     end
   end
 end