Feature: Slack integration (#783)

- Integrations architecture
- Slack integration

app/jobs/hook_job.rb

@@ -0,0 +1,11 @@
+class HookJob < ApplicationJob
+  queue_as :integrations
+
+  def perform(hook, message)
+    return unless hook.slack?
+
+    Integrations::Slack::OutgoingMessageBuilder.perform(hook, message)
+  rescue StandardError => e
+    Raven.capture_exception(e)
+  end
+end