Uploads for admin site settings (#4913)

* Improve OpenGraph tags for about pages

* Add thumbnail admin setting

* Fix error

* Fix up

app/serializers/rest/instance_serializer.rb

@@ -1,8 +1,10 @@
 # frozen_string_literal: true
 
 class REST::InstanceSerializer < ActiveModel::Serializer
+  include RoutingHelper
+
   attributes :uri, :title, :description, :email,
-             :version, :urls, :stats
+             :version, :urls, :stats, :thumbnail
 
   def uri
     Rails.configuration.x.local_domain
@@ -24,6 +26,10 @@ class REST::InstanceSerializer < ActiveModel::Serializer
     Mastodon::Version.to_s
   end
 
+  def thumbnail
+    full_asset_url(instance_presenter.thumbnail.file.url) if instance_presenter.thumbnail
+  end
+
   def stats
     {
       user_count: instance_presenter.user_count,