forum: delete posts when topic is deleted.

Fix it so that when a forum topic is deleted, all posts in the topic are
deleted too. Also make it so that when a forum topic is undeleted, all
posts in it are undeleted too.

Before when a topic was deleted, only the topic itself was marked as
deleted, not the posts inside the topic. This meant that when a spam
topic was deleted, the OP wouldn't be marked as deleted, so any
modreports against it wouldn't be marked as handled.

Also change it so that it's not possible to undelete a post in a deleted
topic, or to delete the OP of a topic without deleting the topic itself.

Finally, add a fix script to delete all active posts in deleted topics,
and to undelete all deleted OPs in active topics.

app/models/forum_topic.rb

@@ -24,12 +24,13 @@ class ForumTopic < ApplicationRecord
   has_many :tag_implications
 
   validates :title, presence: true, length: { maximum: 200 }, if: :title_changed?
-  validates_associated :original_post
   validates :category_id, inclusion: { in: CATEGORIES.keys }
   validates :min_level, inclusion: { in: MIN_LEVELS.values }
 
   accepts_nested_attributes_for :original_post
-  after_update :update_orignal_post
+
+  after_update :update_posts_on_deletion_or_undeletion
+  after_update :update_original_post
   after_save(:if => ->(rec) {rec.is_locked? && rec.saved_change_to_is_locked?}) do |rec|
     ModAction.log("locked forum topic ##{id} (title: #{title})", :forum_topic_lock)
   end
@@ -179,7 +180,14 @@ class ForumTopic < ApplicationRecord
     (response_count / Danbooru.config.posts_per_page.to_f).ceil
   end
 
-  def update_orignal_post
+  # Delete all posts when the topic is deleted. Undelete all posts when the topic is undeleted.
+  def update_posts_on_deletion_or_undeletion
+    if saved_change_to_is_deleted?
+      forum_posts.update!(is_deleted: is_deleted) # XXX depends on current user
+    end
+  end
+
+  def update_original_post
     original_post&.update_columns(:updater_id => updater.id, :updated_at => Time.now)
   end