Feature: Slack integration (#783)

- Integrations architecture
- Slack integration

db/migrate/20200610143132_rename_reference_id.rb

@@ -0,0 +1,5 @@
+class RenameReferenceId < ActiveRecord::Migration[6.0]
+  def change
+    rename_column :conversations, :reference_id, :identifier
+  end
+end