Redesign the landing page, mount public timeline on it (#4122)

* Redesign the landing page, mount public timeline on it

* Adjust the standalone mounted component to the lacking of router

* Adjust auth layout pages to new design

* Fix tests

* Standalone public timeline polling every 5 seconds

* Remove now obsolete translations

* Add responsive design for new landing page

* Address reviews

* Add floating clouds behind frontpage form

* Use access token from public page when available

* Fix mentions and hashtags links, cursor on status content in standalone mode

* Add footer link to source code

* Fix errors on pages that don't embed the component, use classnames

* Fix tests

* Change anonymous autoPlayGif default to false

* When gif autoplay is disabled, hover to play

* Add option to hide the timeline preview

* Slightly improve alt layout

* Add elephant friend to new frontpage

* Display "back to mastodon" in place of "login" when logged in on frontpage

* Change polling time to 3s

app/controllers/admin/settings_controller.rb

@@ -11,8 +11,15 @@ module Admin
       site_terms
       open_registrations
       closed_registrations_message
+      open_deletion
+      timeline_preview
+    ).freeze
+
+    BOOLEAN_SETTINGS = %w(
+      open_registrations
+      open_deletion
+      timeline_preview
     ).freeze
-    BOOLEAN_SETTINGS = %w(open_registrations).freeze
 
     def edit
       @settings = Setting.all_as_records