Change brand color and logotypes (#18592)

- Add rake task for generating Apple/Android icons and favicons from SVG
- Add rake task for generating PNG icons and logos for e-mails from SVG
- Remove obsolete Microsoft icons and configuration
- Remove PWA shortcut icons

app/javascript/styles/application.scss

@@ -6,6 +6,7 @@
 
 @import 'mastodon/reset';
 @import 'mastodon/basics';
+@import 'mastodon/branding';
 @import 'mastodon/containers';
 @import 'mastodon/lists';
 @import 'mastodon/footer';