DEV: configurable public sidebar sections (#20303)

Extension of https://github.com/discourse/discourse/pull/20057

Admin can create a public session visible to everyone. An additional checkbox is displayed for staff members.

db/migrate/20230214044350_add_public_to_sidebar_sections.rb

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