Improved readability of the templates

As per review

app/views/layouts/_startup_css.haml

@@ -2,5 +2,5 @@
 
 %style
   = Rails.application.assets_manifest.find_sources("themes/#{user_application_theme_css_filename}.css").first.to_s.html_safe if user_application_theme_css_filename
-  - if Feature.disabled?(:sidebar_refactor, current_user, default_enabled: :yaml)
+  - if sidebar_refactor_disabled?
     = Rails.application.assets_manifest.find_sources("startup/startup-#{startup_filename}.css").first.to_s.html_safe

app/views/layouts/application.html.haml

 - page_classes = page_class << @html_class
 - page_classes = page_classes.flatten.compact
+- body_classes = [user_application_theme, user_tab_width, @body_class, client_class_list]
+- body_classes << 'sidebar-refactoring' if sidebar_refactor_enabled?
 
 !!! 5
 %html{ lang: I18n.locale, class: page_classes }
   = render "layouts/head"
-  %body{ class: [user_application_theme, user_tab_width, @body_class, client_class_list, ('sidebar-refactoring' if Feature.enabled?(:sidebar_refactor, current_user, default_enabled: :yaml))], data: body_data }
+  %body{ class: body_classes, data: body_data }
     = render "layouts/init_auto_complete" if @gfm_form
     = render "layouts/init_client_detection_flags"
     = render 'peek/bar'