DEV: Compile splash screen JS with ember-cli before inlining (#18150)

This lets us use all our normal JS tooling like prettier, esline and babel on the splash screen JS. At runtime the JS file is read and inlined into the HTML. This commit also switches us to use a CSP hash rather than a nonce for the splash screen.

app/helpers/application_helper.rb

@@ -128,10 +128,6 @@ module ApplicationHelper
     path
   end
 
-  def self.splash_screen_nonce
-    @splash_screen_nonce ||= SecureRandom.hex
-  end
-
   def preload_script(script)
     scripts = [script]