Change settings area to be separated into categories in admin UI (#19407)

app/controllers/admin/settings/about_controller.rb

app/controllers/admin/settings/appearance_controller.rb

app/controllers/admin/settings/branding_controller.rb

...ontrollers/admin/settings/content_retention_controller.rb

app/controllers/admin/settings/discovery_controller.rb

app/controllers/admin/settings/registrations_controller.rb

app/controllers/admin/settings_controller.rb

app/controllers/admin/site_uploads_controller.rb

app/helpers/admin/settings_helper.rb

app/javascript/styles/mastodon/admin.scss

app/javascript/styles/mastodon/components.scss

app/javascript/styles/mastodon/forms.scss

app/models/form/admin_settings.rb

app/serializers/rest/extended_description_serializer.rb

app/views/admin/settings/about/show.html.haml

app/views/admin/settings/appearance/show.html.haml

app/views/admin/settings/branding/show.html.haml

app/views/admin/settings/content_retention/show.html.haml

app/views/admin/settings/discovery/show.html.haml

app/views/admin/settings/edit.html.haml