wiki_pages/show: refactor post previews.

* Increase number of previews from 4 to 8.
* Eliminate wiki page post set classes.

app/models/tag.rb

@@ -914,6 +914,10 @@ class Tag < ApplicationRecord
     return false
   end
 
+  def posts
+    Post.tag_match(name)
+  end
+
   include ApiMethods
   include CountMethods
   include CategoryMethods

app/models/wiki_page.rb

@@ -237,10 +237,6 @@ class WikiPage < ApplicationRecord
     self.dtext_links = DtextLink.new_from_dtext(body)
   end
 
-  def post_set
-    @post_set ||= PostSets::WikiPage.new(title, 1, 4)
-  end
-
   def presenter
     @presenter ||= WikiPagePresenter.new(self)
   end