DEV: configurable custom sidebar sections (#20057)

Allows users to configure their own custom sidebar sections with links withing Discourse instance. Links can be passed as relative path, for example "/tags" or full URL.

Only path is saved in DB, so when Discourse domain is changed, links will be still valid.

Feature is hidden behind SiteSetting.enable_custom_sidebar_sections. This hidden setting determines the group which members have access to this new feature.

lib/guardian/sidebar_guardian.rb

@@ -0,0 +1,11 @@
+# frozen_string_literal: true
+
+module SidebarGuardian
+  def can_edit_sidebar_section?(sidebar_section)
+    is_my_own?(sidebar_section)
+  end
+
+  def can_delete_sidebar_section?(sidebar_section)
+    is_my_own?(sidebar_section)
+  end
+end