dtext links: fix links from mod-only forum posts being exposed.

Fix links from mod-only forum posts being publicly visible in the
/dtext_links page.

app/models/dtext_link.rb

@@ -11,6 +11,12 @@ class DtextLink < ApplicationRecord
   scope :wiki_page, -> { where(model_type: "WikiPage") }
   scope :forum_post, -> { where(model_type: "ForumPost") }
 
+  def self.visible(user)
+    # XXX the double negation is to prevent postgres from choosing a bad query
+    # plan (it doesn't know that most forum posts aren't mod-only posts).
+    wiki_page.or(forum_post.where.not(model_id: ForumPost.not_visible(user)))
+  end
+
   def self.model_types
     %w[WikiPage ForumPost]
   end

app/models/forum_post.rb

@@ -40,6 +40,10 @@ class ForumPost < ApplicationRecord
       where(topic_id: ForumTopic.visible(user))
     end
 
+    def not_visible(user)
+      where.not(topic_id: ForumTopic.visible(user))
+    end
+
     def wiki_link_matches(title)
       where(id: DtextLink.forum_post.wiki_link.where(link_target: WikiPage.normalize_title(title)).select(:model_id))
     end