Feature: Slack integration (#783)

- Integrations architecture
- Slack integration

app/models/integrations/hook.rb

@@ -0,0 +1,36 @@
+# == Schema Information
+#
+# Table name: integrations_hooks
+#
+#  id           :bigint           not null, primary key
+#  access_token :string
+#  hook_type    :integer          default("account")
+#  settings     :text
+#  status       :integer          default("disabled")
+#  created_at   :datetime         not null
+#  updated_at   :datetime         not null
+#  account_id   :integer
+#  app_id       :string
+#  inbox_id     :integer
+#  reference_id :string
+#
+class Integrations::Hook < ApplicationRecord
+  validates :account_id, presence: true
+  validates :app_id, presence: true
+
+  enum status: { disabled: 0, enabled: 1 }
+
+  belongs_to :account
+  belongs_to :inbox, optional: true
+  has_secure_token :access_token
+
+  enum hook_type: { account: 0, inbox: 1 }
+
+  def app
+    @app ||= Integrations::App.find(id: app_id)
+  end
+
+  def slack?
+    app_id == 'cw_slack'
+  end
+end