Change about page to be mounted in the web UI (#19345)

app/models/extended_description.rb

@@ -0,0 +1,15 @@
+# frozen_string_literal: true
+
+class ExtendedDescription < ActiveModelSerializers::Model
+  attributes :updated_at, :text
+
+  def self.current
+    custom = Setting.find_by(var: 'site_extended_description')
+
+    if custom&.value.present?
+      new(text: custom.value, updated_at: custom.updated_at)
+    else
+      new
+    end
+  end
+end