Escape story fields and clean story HTML with HTML Purifier.

Add HTML Purifier 4.19.1. Stories are purified when they are saved and
when they are shown.

lib/htmlpurifier-4.19.1/library/HTMLPurifier/ConfigSchema/schema/URI.AllowedSymbols.txt

@@ -0,0 +1,7 @@
+URI.AllowedSymbols
+TYPE: string/null
+DEFAULT: '!$&\'()*+,;='
+--DESCRIPTION--
+If a system permits templated URLs, then the URI encoder may need extra
+hints about which symbols to preserve.
+--# vim: et sw=4 sts=4