Efficiency: Entity handling

Previously, used a regexp to find and convert named entities in the content.
Now use a more efficient algorithm.
Similar tweak for converting NCRs before checking whether text is valid utf-8.

lib/wiki_content.rb

@@ -7,6 +7,8 @@ require_dependency 'chunks/literal'
 require 'chunks/nowiki'
 require 'sanitize'
 
+include Sanitize
+
 # Wiki content is just a string that can process itself with a chain of
 # actions. The actions can modify wiki content so that certain parts of
 # it are protected from being rendered by later actions.