fixes #4079: Allow deleted forum items to be seen with the is_deleted parameter

app/models/forum_post.rb

@@ -162,8 +162,8 @@ class ForumPost < ApplicationRecord
     (creator_id == user.id || user.is_moderator?) && visible?(user)
   end
 
-  def visible?(user)
-    user.is_moderator? || (topic.visible?(user) && !is_deleted?)
+  def visible?(user, show_deleted_posts = false)
+    user.is_moderator? || (topic.visible?(user) && (show_deleted_posts || !is_deleted?))
   end
 
   def update_topic_updated_at_on_create

app/views/forum_posts/_forum_post.html.erb

@@ -1,6 +1,4 @@
-<%- # original_forum_post_id: used to accelerate #is_original_post? calls %>
-
-<% if forum_post.visible?(CurrentUser.user) %>
+<% if forum_post.visible?(CurrentUser.user, ActiveModel::Type::Boolean.new.cast(params.dig(:search, :is_deleted))) %>
   <article class="forum-post" id="forum_post_<%= forum_post.id %>" data-forum-post-id="<%= forum_post.id %>" data-creator="<%= forum_post.creator.name %>">
     <div class="author">
       <h4>