Update Rails to version 5.1.1 (#3121)

* Update rails to version 5.1.1

* Run `rails app:update`

* Remove the override of polymorphic activity relationship

* Silence warning about otp_secret attribute being unknown to rails

* We will only introduce form_with where we want to use remote data

config/environments/development.rb

@@ -16,8 +16,9 @@ Rails.application.configure do
   if Rails.root.join('tmp/caching-dev.txt').exist?
     config.action_controller.perform_caching = true
 
+    config.cache_store = :memory_store
     config.public_file_server.headers = {
-      'Cache-Control' => 'public, max-age=172800',
+      'Cache-Control' => "public, max-age=#{2.days.seconds.to_i}"
     }
   else
     config.action_controller.perform_caching = false
@@ -32,6 +33,7 @@ Rails.application.configure do
 
   # Don't care if the mailer can't send.
   config.action_mailer.raise_delivery_errors = false
+
   config.action_mailer.perform_caching = false
 
   # Print deprecation notices to the Rails logger.