fixes #1450

Additionally:
* Adds basic negated id metatag.
* Post set presenter now uses the post set's tag string, not the
parameter from the URL.

app/logical/post_query_builder.rb

@@ -207,6 +207,10 @@ class PostQueryBuilder
       has_constraints!
     end
 
+    if q[:post_id_negated]
+      relation = relation.where("posts.id <> ?", q[:post_id_negated])
+    end
+
     if q[:parent] == "none"
       relation = relation.where("posts.parent_id IS NULL")
     elsif q[:parent_neg] == "none" || q[:parent] == "any"