Upgrade to Rails 5.2

All the code in the `bin/` and the `config/` folder has been generated
running `rake app:update`, except the `escape_javascript_fix` file,
which we've removed since the code there is already included in Rails
5.2.

config/initializers/application_controller_renderer.rb

@@ -1,6 +1,8 @@
 # Be sure to restart your server when you modify this file.
 
-# ApplicationController.renderer.defaults.merge!(
-#   http_host: 'example.org',
-#   https: false
-# )
+# ActiveSupport::Reloader.to_prepare do
+#   ApplicationController.renderer.defaults.merge!(
+#     http_host: 'example.org',
+#     https: false
+#   )
+# end