Add new first-time tutorial (#9531)

* Prepare to load onboarding as a full page

* Update the first-time introduction

* Improve responsive design

* Replace speech bubble with logo

* Increase text size and reword first paragraph

app/javascript/mastodon/features/ui/util/async-components.js

@@ -102,10 +102,6 @@ export function Mutes () {
   return import(/* webpackChunkName: "features/mutes" */'../../mutes');
 }
 
-export function OnboardingModal () {
-  return import(/* webpackChunkName: "modals/onboarding_modal" */'../components/onboarding_modal');
-}
-
 export function MuteModal () {
   return import(/* webpackChunkName: "modals/mute_modal" */'../components/mute_modal');
 }