chore: Improve search, list performance of contact/conversation APIs (#2696)

db/migrate/20210723094412_add_index_to_conversations.rb

@@ -0,0 +1,6 @@
+class AddIndexToConversations < ActiveRecord::Migration[6.0]
+  def change
+    add_index :conversations, [:status, :account_id]
+    add_index :conversations, [:assignee_id, :account_id]
+  end
+end

db/migrate/20210723095657_add_index_to_contacts.rb

@@ -0,0 +1,5 @@
+class AddIndexToContacts < ActiveRecord::Migration[6.0]
+  def change
+    add_index :contacts, [:phone_number, :account_id]
+  end
+end