Uploads for admin site settings (#4913)

* Improve OpenGraph tags for about pages

* Add thumbnail admin setting

* Fix error

* Fix up

app/presenters/instance_presenter.rb

@@ -35,4 +35,8 @@ class InstancePresenter
   def source_url
     Mastodon::Version.source_url
   end
+
+  def thumbnail
+    @thumbnail ||= Rails.cache.fetch('site_uploads/thumbnail') { SiteUpload.find_by(var: 'thumbnail') }
+  end
 end