Replace tutorial modal with welcome e-mail (#6273)

* Remove onboarding modal

* Welcome e-mail

* Send welcome e-mail after confirmation

* Remove obsolete translations

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

@@ -94,10 +94,6 @@ 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');
 }