discord: add initial slash command integration.

Add initial support for the `/count <tags>` and `/posts <tags>` slash commands.

Slash commands are basically like webhooks; we register a command, and
when anybody types that command in Discord, Discord sends us a HTTP
request that we respond to.

* https://discord.com/developers/docs/interactions/slash-commands
* https://support.discord.com/hc/en-us/articles/1500000368501-Slash-Commands-FAQ

config/danbooru_default_config.rb

@@ -402,6 +402,29 @@ module Danbooru
     def discord_webhook_secret
     end
 
+    # Settings used for Discord slash commands.
+    #
+    # * Go to https://discord.com/developers/applications
+    # * Create an application.
+    # * Copy the client ID and public key.
+    # * Create a bot user.
+    # * Copy the bot token.
+    # * Go to the OAuth2 page, select the `bot` and `applications.commands`
+    #   scopes, and the `Administrator` permission, then follow the oauth2
+    #   link to add the bot to the Discord server.
+    def discord_application_client_id
+    end
+
+    def discord_application_public_key
+    end
+
+    def discord_bot_token
+    end
+
+    # The ID of the Discord server to register slash commands for.
+    def discord_guild_id
+    end
+
     # you should override this
     def email_key
       "zDMSATq0W3hmA5p3rKTgD"