Preload CSS and JavaScript files

This is the default setting in Rails 6.1, and generates an extra tag in
the HTML which tells the browser to download and cache these files as
soon as possible, even before they're needed.

This might not be that relevant in our application, since on most pages
we only generate one CSS and one JS file. But it might make it easier to
move the `javascript_include_tag` statement to the bottom of the page in
the future if we detect that doing so increases performance.

config/initializers/new_framework_defaults_6_1.rb

@@ -49,4 +49,4 @@ Rails.application.config.action_mailer.deliver_later_queue_name = nil
 
 # Generate a `Link` header that gives a hint to modern browsers about
 # preloading assets when using `javascript_include_tag` and `stylesheet_link_tag`.
-# Rails.application.config.action_view.preload_links_header = true
+Rails.application.config.action_view.preload_links_header = true