Feature: Slack integration (#783)

- Integrations architecture
- Slack integration

db/migrate/20200430163438_create_integrations_hooks.rb

@@ -0,0 +1,15 @@
+class CreateIntegrationsHooks < ActiveRecord::Migration[6.0]
+  def change
+    create_table :integrations_hooks do |t|
+      t.integer :status, default: 0
+      t.integer :inbox_id
+      t.integer :account_id
+      t.string :app_id
+      t.text :settings
+      t.integer :hook_type, default: 0
+      t.string :reference_id
+      t.string :access_token
+      t.timestamps
+    end
+  end
+end