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/mastodon/branding.scss

@@ -0,0 +1,3 @@
+.logo {
+  color: $primary-text-color;
+}