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/views/about/_features.html.haml

@@ -0,0 +1,25 @@
+.features-list
+  .features-list__row
+    .text
+      %h6= t 'about.features.real_conversation_title'
+      = t 'about.features.real_conversation_body'
+    .visual
+      = fa_icon 'fw comments'
+  .features-list__row
+    .text
+      %h6= t 'about.features.not_a_product_title'
+      = t 'about.features.not_a_product_body'
+    .visual
+      = fa_icon 'fw users'
+  .features-list__row
+    .text
+      %h6= t 'about.features.within_reach_title'
+      = t 'about.features.within_reach_body'
+    .visual
+      = fa_icon 'fw mobile'
+  .features-list__row
+    .text
+      %h6= t 'about.features.humane_approach_title'
+      = t 'about.features.humane_approach_body'
+    .visual
+      = fa_icon 'fw leaf'