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/test.rb

@@ -12,10 +12,10 @@ Rails.application.configure do
   # preloads Rails for running tests, you may have to set it to true.
   config.eager_load = false
 
-  # Configure static file server for tests with Cache-Control for performance.
+  # Configure public file server for tests with Cache-Control for performance.
   config.public_file_server.enabled = true
   config.public_file_server.headers = {
-    'Cache-Control' => 'public, max-age=3600'
+    'Cache-Control' => "public, max-age=#{1.hour.seconds.to_i}"
   }
   config.assets.digest = false
 
@@ -28,12 +28,12 @@ Rails.application.configure do
 
   # Disable request forgery protection in test environment.
   config.action_controller.allow_forgery_protection = false
+  config.action_mailer.perform_caching = false
 
   # Tell Action Mailer not to deliver emails to the real world.
   # The :test delivery method accumulates sent emails in the
   # ActionMailer::Base.deliveries array.
   config.action_mailer.delivery_method = :test
-  config.action_mailer.perform_caching = false
 
   # Print deprecation notices to the stderr.
   config.active_support.deprecation = :stderr