Redesign landing page (again) (#6486)

* Redesign landing page (again)

* Move login form in small version to the right column

* Display closed registrations message

* Add site setting for the hero image

* Fix test

* Increase spacing, maximum width, change call to action section

app/presenters/instance_presenter.rb

@@ -39,4 +39,8 @@ class InstancePresenter
   def thumbnail
     @thumbnail ||= Rails.cache.fetch('site_uploads/thumbnail') { SiteUpload.find_by(var: 'thumbnail') }
   end
+
+  def hero
+    @hero ||= Rails.cache.fetch('site_uploads/hero') { SiteUpload.find_by(var: 'hero') }
+  end
 end