Feature: Slack integration (#783)

- Integrations architecture
- Slack integration

spec/factories/integrations/hooks.rb

@@ -0,0 +1,12 @@
+FactoryBot.define do
+  factory :integrations_hook, class: 'Integrations::Hook' do
+    status { 1 }
+    inbox_id { 1 }
+    account_id { 1 }
+    app_id { 'cw_slack' }
+    settings { 'MyText' }
+    hook_type { 1 }
+    access_token { SecureRandom.hex }
+    reference_id { SecureRandom.hex }
+  end
+end