feat(i18n): enable localization by query param

Providing a query param ("locale") will enable localization. A language picker will be shown once
localization is activated. Locale is stored in a cookie "locale".

spec/views/layouts/_header_spec.rb

@@ -5,6 +5,7 @@ describe 'layouts/_header.html.haml', type: :view do
     allow(view).to receive(:multiple_devise_profile_connect?).and_return(false)
     allow(view).to receive(:instructeur_signed_in?).and_return((profile == :instructeur))
     allow(view).to receive(:current_instructeur).and_return(current_instructeur)
+    allow(view).to receive(:localization_enabled?).and_return(false)
 
     if user
       sign_in user

spec/views/layouts/procedure_context.html.haml_spec.rb

@@ -5,6 +5,7 @@ describe 'layouts/procedure_context.html.haml', type: :view do
   before do
     allow(view).to receive(:instructeur_signed_in?).and_return(false)
     allow(view).to receive(:administrateur_signed_in?).and_return(false)
+    allow(view).to receive(:localization_enabled?).and_return(false)
   end
 
   subject do