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/styles/application.scss

@@ -16,6 +16,7 @@
 @import 'mastodon/stream_entries';
 @import 'mastodon/boost';
 @import 'mastodon/components';
+@import 'mastodon/introduction';
 @import 'mastodon/modal';
 @import 'mastodon/emoji_picker';
 @import 'mastodon/about';