add favgroup excerpt

app/logical/post_sets/post.rb

@@ -55,6 +55,18 @@ module PostSets
       ::Pool.find_by_name(pool_name)
     end
 
+    def favgroup_name
+      tag_string.match(/^favgroup:(\S+)$/i).try(:[], 1)
+    end
+
+    def has_favgroup?
+      is_single_tag? && favgroup_name && favgroup
+    end
+
+    def favgroup
+      ::FavoriteGroup.find_by_name(favgroup_name)
+    end
+
     def has_deleted?
       tag_string !~ /status/ && ::Post.tag_match("#{tag_string} status:deleted").exists?
     end