Use text instead of image for logo

- "it is good design practice to use actual text that is styled with CSS rather than image-based text presentation" (https://www.w3.org/WAI/tutorials/images/textual)

- mm markup que https://www.gov.uk/ et que https://www.w3.org/WAI/tutorials/images/decorative/#decorative-image-as-part-of-a-text-link

app/views/layouts/_new_header.haml

@@ -10,7 +10,10 @@
       - if params[:controller] == 'users/commencer'
         = link_to 'Revenir en arrière', url_for(:back), class: "button", title: "Revenir sur le site de mon administration"
       - else
-        = link_to '', root_path_for_profile(nav_bar_profile), class: "header-logo", title: "Revenir à l’accueil"
+        = link_to root_path_for_profile(nav_bar_profile), class: 'header-logo', title: "Aller à la page d'accueil de demarches-simplifiees.fr" do
+          = image_tag 'marianne.svg', alt: '', width: 65
+          %span.big.site-title> demarches-simplifiees.fr
+          %span.small.site-title> d-s.fr
 
       - if nav_bar_profile == :instructeur && instructeur_signed_in?
         - current_url = request.path_info