Feature: Slack integration (#783)

- Integrations architecture
- Slack integration

lib/integrations/slack/outgoing_message_builder.rb

@@ -0,0 +1,50 @@
+class Integrations::Slack::OutgoingMessageBuilder
+  attr_reader :hook, :message
+
+  def self.perform(hook, message)
+    new(hook, message).perform
+  end
+
+  def initialize(hook, message)
+    @hook = hook
+    @message = message
+  end
+
+  def perform
+    send_message
+    update_reference_id
+  end
+
+  private
+
+  def conversation
+    @conversation ||= message.conversation
+  end
+
+  def contact
+    @contact ||= conversation.contact
+  end
+
+  def send_message
+    @slack_message = slack_client.chat_postMessage(
+      channel: hook.reference_id,
+      text: message.content,
+      username: contact.try(:name),
+      thread_ts: conversation.identifier
+    )
+  end
+
+  def update_reference_id
+    return if conversation.identifier
+
+    conversation.identifier = @slack_message['ts']
+    conversation.save!
+  end
+
+  def slack_client
+    Slack.configure do |config|
+      config.token = hook.access_token
+    end
+    Slack::Web::Client.new
+  end
+end