UX: Introduces a splash screen behind a hidden site setting (#17094)

This PR introduces a new hidden site setting that allows admins to display a splash screen while site assets load.

The splash screen can be enabled via the `splash_screen` hidden site setting.

This is what the splash screen currently looks like

5ceb72f085.mp4

Once site assets load, the splash screen is automatically removed.

To control the loading text that shows in the splash screen, you can change the preloader_text translation string in admin > customize > text

spec/requests/application_controller_spec.rb

@@ -529,6 +529,28 @@ RSpec.describe ApplicationController do
     end
   end
 
+  describe "splash_screen" do
+    let(:admin) { Fabricate(:admin) }
+
+    before do
+      admin
+    end
+
+    it 'adds a preloader splash screen when enabled' do
+      get '/'
+
+      expect(response.status).to eq(200)
+      expect(response.body).not_to include("d-splash")
+
+      SiteSetting.splash_screen = true
+
+      get '/'
+
+      expect(response.status).to eq(200)
+      expect(response.body).to include("d-splash")
+    end
+  end
+
   describe 'Delegated auth' do
     let :public_key do
       <<~TXT