Change about page to be mounted in the web UI (#19345)

app/javascript/styles/application.scss

@@ -2,7 +2,6 @@
 @import 'mastodon/variables';
 @import 'fonts/roboto';
 @import 'fonts/roboto-mono';
-@import 'fonts/montserrat';
 
 @import 'mastodon/reset';
 @import 'mastodon/basics';
@@ -10,7 +9,6 @@
 @import 'mastodon/containers';
 @import 'mastodon/lists';
 @import 'mastodon/footer';
-@import 'mastodon/compact_header';
 @import 'mastodon/widgets';
 @import 'mastodon/forms';
 @import 'mastodon/accounts';