DEV: Use performance timings to as a baseline for the splash take 2 (#17284)

We previously relied on CSS animation-delay for the splash. This means that we can get inconsistent results based on device/network conditions.

This PR moves us to a more consistent timing based on {request time + 2 seconds}

Internal topic: /t/65378/65

app/helpers/application_helper.rb

@@ -135,6 +135,10 @@ module ApplicationHelper
     path
   end
 
+  def self.splash_screen_nonce
+    @splash_screen_nonce ||= SecureRandom.hex
+  end
+
   def preload_script(script)
     scripts = [script]