FEAT: Allow admin delete user's associated accounts (#29018)

This commit introduces a feature that allows an admin to delete a user's
associated account. After deletion, a log will be recorded in staff
actions.

ref=t/136675

config/routes.rb

@@ -162,6 +162,7 @@ Discourse::Application.routes.draw do
         put "disable_second_factor"
         delete "sso_record"
         get "similar-users.json" => "users#similar_users"
+        put "delete_associated_accounts"
       end
       get "users/:id.json" => "users#show", :defaults => { format: "json" }
       get "users/:id/:username" => "users#show",