Reduce server load caused by anonymous viewing. (#9059)

Do not start a session if the current user is not logged in for public-facing pages.

Mark pages that don't care about sessions as publicly cacheable.

Keep the max age as 0 so proxies and browsers will still try to retrieve an updated version but can still fall back to the stale version if the site is down or too slow.

Fixes #9035.

app/controllers/application_controller.rb

@@ -151,6 +151,11 @@ class ApplicationController < ActionController::Base
     response.headers['Vary'] = 'Accept'
   end
 
+  def mark_cacheable!
+    skip_session!
+    expires_in 0, public: true
+  end
+
   def skip_session!
     request.session_options[:skip] = true
   end