Split requests made by Danbooru::Http into either internal or external
requests. Internal requests are API calls to internal services run by
Danbooru. External requests are requests to external websites, for
example fetching sources or downloading files. External requests may use
a HTTP proxy if one is configured. Internal requests don't.
Fixes a few source extractors not using the HTTP proxy for certain API calls.
* 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.
