Update mod queue post previews for consistency with rest of site

Fixes display of borders on the previews and such. Also fix the yellow
duplicate background not always working because Post#has_dup_tag? didn't
return a boolean.

app/models/post.rb

@@ -636,7 +636,7 @@ class Post < ActiveRecord::Base
     end
 
     def has_tag?(tag)
-      tag_string =~ /(?:^| )#{tag}(?:$| )/
+      !!(tag_string =~ /(?:^| )#{tag}(?:$| )/)
     end
 
     def has_dup_tag?