fix: Add index in conversation and reporting event (#4577)

Fixes chatwoot/product#422

db/migrate/20220428101325_add_index_to_conversation_and_reporting_event.rb

@@ -0,0 +1,6 @@
+class AddIndexToConversationAndReportingEvent < ActiveRecord::Migration[6.1]
+  def change
+    add_index :conversations, :last_activity_at
+    add_index :reporting_events, :conversation_id
+  end
+end