FEATURE: allow external links in custom sidebar sections (#20503)

Originally, only Discourse site links were available. After feedback, it was decided to extend this feature to external URLs.

/t/93491

db/migrate/20230303015952_add_external_to_sidebar_urls.rb

@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+class AddExternalToSidebarUrls < ActiveRecord::Migration[7.0]
+  def change
+    add_column :sidebar_urls, :external, :boolean, default: false, null: false
+  end
+end