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.

config/routes.rb

@@ -1588,6 +1588,8 @@ Discourse::Application.routes.draw do
     put "user-status" => "user_status#set"
     delete "user-status" => "user_status#clear"
 
+    resources :sidebar_sections, only: %i[create update destroy]
+
     get "*url", to: "permalinks#show", constraints: PermalinkConstraint.new
   end
 end