For small thumbnails, use 180x180 thumbnails scaled down to 150x150.
This is so we can get rid of 150x150 images and just use 180x180 for
both small and medium size thumbnails.
Also fix RSS feeds, XML sitemaps, and Discord embeds to use 360x360
thumbnails instead of 150x150 thumbnails.
* Add a `DiscordSlashCommand.register_slash_commands!` method to register
all slash commands with the Discord API.
* Allow registering global commands.
* Refactor slash commands to use class attributes for the command
name, description, and options.
