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/views/layouts/admin.html.haml

@@ -8,12 +8,12 @@
       .sidebar-wrapper__inner
         .sidebar
           = link_to root_path do
-            = image_pack_tag 'logo.svg', class: 'logo', alt: 'Mastodon'
+            = render_logo
 
           .sidebar__toggle
             .sidebar__toggle__logo
               = link_to root_path do
-                = svg_logo_full
+                = logo_as_symbol(:wordmark)
 
             = link_to '#', class: 'sidebar__toggle__icon' do
               = fa_icon 'bars'