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

spec/views/about/show.html.haml_spec.rb

@@ -16,6 +16,9 @@ describe 'about/show.html.haml', without_verify_partial_doubles: true do
                                 source_url: 'https://github.com/tootsuite/mastodon',
                                 open_registrations: false,
                                 thumbnail: nil,
+                                hero: nil,
+                                user_count: 0,
+                                status_count: 0,
                                 closed_registrations_message: 'yes')
     assign(:instance_presenter, instance_presenter)
     render