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

app/logical/discord_slash_command.rb

@@ -0,0 +1,117 @@
+class DiscordSlashCommand
+  class WebhookVerificationError < StandardError; end
+
+  COMMANDS = {
+    count: DiscordSlashCommand::CountCommand,
+    posts: DiscordSlashCommand::PostsCommand,
+  }
+
+  # https://discord.com/developers/docs/interactions/slash-commands#interaction-interactiontype
+  module InteractionType
+    Ping = 1
+    ApplicationCommand = 2
+  end
+
+  # https://discord.com/developers/docs/interactions/slash-commands#applicationcommandoptiontype
+  module ApplicationCommandOptionType
+    String = 3
+  end
+
+  attr_reader :data, :discord
+
+  # `data` is the the interaction data sent to us by Discord for the command.
+  # https://discord.com/developers/docs/interactions/slash-commands#interaction
+  def initialize(data: {}, discord: DiscordApiClient.new)
+    @data = data
+    @discord = discord
+  end
+
+  # The name of the slash command.
+  def name
+    raise NotImplementedError
+  end
+
+  # A description of the slash command.
+  def description
+    raise NotImplementedError
+  end
+
+  # The parameters of the slash command.
+  # https://discord.com/developers/docs/interactions/slash-commands#applicationcommandoption
+  def options
+    []
+  end
+
+  # Should return the response to the command.
+  def call
+    # respond_with("message")
+    raise NotImplementedError
+  end
+
+  concerning :HelperMethods do
+    # The parameters passed to the command. A hash.
+    def params
+      @params ||= data.dig(:data, :options).map do |opt|
+        [opt[:name], opt[:value]]
+      end.to_h.with_indifferent_access
+    end
+
+    # https://discord.com/developers/docs/interactions/slash-commands#responding-to-an-interaction
+    # https://discord.com/developers/docs/interactions/slash-commands#interaction-response
+    def respond_with(content = nil, type: 4, **options)
+      {
+        type: type,
+        data: {
+          content: content,
+          **options
+        }
+      }
+    end
+
+    # Register the command with the Discord API (replacing it if it already exists).
+    # https://discord.com/developers/docs/interactions/slash-commands#registering-a-command
+    def register_slash_command
+      discord.register_slash_command(name: name, description: description, options: options)
+    end
+  end
+
+  concerning :WebhookMethods do
+    class_methods do
+      # Called when we receive a command from Discord. Instantiates a
+      # DiscordSlashCommand and calls the `call` method.
+      # https://discord.com/developers/docs/interactions/slash-commands#interaction
+      def receive_webhook(request)
+        data = verify_request!(request)
+
+        case data[:type]
+        when InteractionType::Ping
+          { type: InteractionType::Ping }
+        when InteractionType::ApplicationCommand
+          name = data.dig(:data, :name)
+          klass = COMMANDS.fetch(name&.to_sym)
+          klass.new(data: data).call
+        else
+          raise NotImplementedError, "unknown Discord interaction type #{data[:type]}"
+        end
+      end
+
+      # https://discord.com/developers/docs/interactions/slash-commands#security-and-authorization
+      def verify_request!(request, public_key: Danbooru.config.discord_application_public_key)
+        timestamp = request.headers["X-Signature-Timestamp"].to_s
+        signature = request.headers["X-Signature-Ed25519"].to_s
+        signature_bytes = [signature].pack("H*")
+
+        body = request.body.read
+        message = timestamp + body
+
+        public_key_bytes = [public_key].pack("H*")
+        verify_key = Ed25519::VerifyKey.new(public_key_bytes)
+
+        verify_key.verify(signature_bytes, message)
+        JSON.parse(body).with_indifferent_access
+      rescue Ed25519::VerifyError
+        raise WebhookVerificationError
+      end
+    end
+  end
+end