Remove StartupCSS Checks

Updated for test setup, restructured startup setup

Restructured to have a feature flag in place
for startup_css

app/helpers/application_helper.rb

@@ -236,11 +236,11 @@ module ApplicationHelper
   end
 
   def use_startup_css?
-    params[:startup_css] == 'true' || cookies['startup_css'] == 'true'
+    Feature.enabled?(:startup_css)
   end
 
   def stylesheet_link_tag_defer(path)
-    if use_startup_css?
+    if use_startup_css? && !Rails.env.test?
       stylesheet_link_tag(path, media: "print")
     else
       stylesheet_link_tag(path, media: "all")

app/views/layouts/_head.html.haml

@@ -49,9 +49,7 @@
 
   = favicon_link_tag favicon, id: 'favicon', data: { original_href: favicon }, type: 'image/png'
 
-  - if use_startup_css?
-    %style{ type: "text/css" }
-      = Rails.application.assets_manifest.find_sources('startup/startup-general.css').first.to_s.html_safe
+  = render 'layouts/startup_css'
   - if user_application_theme == 'gl-dark'
     = stylesheet_link_tag_defer "application_dark"
   - else
@@ -61,7 +59,7 @@
 
   = stylesheet_link_tag_defer "highlight/themes/#{user_color_scheme}"
 
-  = render 'layouts/startup_css'
+  = render 'layouts/startup_css_activation'
 
   = Gon::Base.render_data(nonce: content_security_policy_nonce)
 

app/views/layouts/_startup_css.haml

 - return unless use_startup_css?
 
-= javascript_tag nonce: true do
-  :plain
-    document.querySelectorAll('link[media="print"]').forEach(linkTag => {
-      linkTag.addEventListener('load', function() {this.media='all'}, {once: true});
-    })
+%style{ type: "text/css" }
+  = Rails.application.assets_manifest.find_sources('startup/startup-general.css').first.to_s.html_safe

app/views/layouts/_startup_css_activation.haml

+- return unless use_startup_css?
+
+= javascript_tag nonce: true do
+  :plain
+    document.querySelectorAll('link[media="print"]').forEach(linkTag => {
+      linkTag.addEventListener('load', function() {this.media='all'}, {once: true});
+    })

config/feature_flags/development/startup_css.yml

+---
+name: startup_css
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/39713
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/238718
+group: group::editor
+type: development
+default_enabled: false