Feature: Slack integration (#783)

- Integrations architecture
- Slack integration

spec/models/inbox_spec.rb

@@ -29,6 +29,8 @@ RSpec.describe Inbox do
     it { is_expected.to have_many(:webhooks).dependent(:destroy) }
 
     it { is_expected.to have_many(:events) }
+
+    it { is_expected.to have_many(:hooks) }
   end
 
   describe '#add_member' do