UX: improve user delete error message & return correct post count. (#13282)

Post count was incorrect on admin page causing confusion when admins attempted to delete users.

app/controllers/admin/users_controller.rb

@@ -420,7 +420,7 @@ class Admin::UsersController < Admin::AdminController
       rescue UserDestroyer::PostsExistError
         render json: {
           deleted: false,
-          message: "User #{user.username} has #{user.post_count} posts, so they can't be deleted."
+          message: I18n.t("user.cannot_delete_has_posts", username: user.username, post_count: user.posts.joins(:topic).count),
         }, status: 403
       end
     end