Revamp entity decoding to be more like HTML5.

See %Core.LegacyEntityDecoder for more details.

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>

library/HTMLPurifier/ConfigSchema/schema/Core.LegacyEntityDecoder.txt

@@ -0,0 +1,36 @@
+Core.LegacyEntityDecoder
+TYPE: bool
+VERSION: 4.9.0
+DEFAULT: false
+--DESCRIPTION--
+<p>
+    Prior to HTML Purifier 4.9.0, entities were decoded by performing
+    a global search replace for all entities whose decoded versions
+    did not have special meanings under HTML, and replaced them with
+    their decoded versions.  We would match all entities, even if they did
+    not have a trailing semicolon, but only if there weren't any trailing
+    alphanumeric characters.
+</p>
+<table>
+<tr><th>Original</th><th>Text</th><th>Attribute</th></tr>
+<tr><td>&amp;yen;</td><td>&yen;</td><td>&yen;</td></tr>
+<tr><td>&amp;yen</td><td>&yen;</td><td>&yen;</td></tr>
+<tr><td>&amp;yena</td><td>&amp;yena</td><td>&amp;yena</td></tr>
+<tr><td>&amp;yen=</td><td>&yen;=</td><td>&yen;=</td></tr>
+</table>
+<p>
+    In HTML Purifier 4.9.0, we changed the behavior of entity parsing
+    to match entities that had missing trailing semicolons in less
+    cases, to more closely match HTML5 parsing behavior:
+</p>
+<table>
+<tr><th>Original</th><th>Text</th><th>Attribute</th></tr>
+<tr><td>&amp;yen;</td><td>&yen;</td><td>&yen;</td></tr>
+<tr><td>&amp;yen</td><td>&yen;</td><td>&yen;</td></tr>
+<tr><td>&amp;yena</td><td>&yen;a</td><td>&amp;yena</td></tr>
+<tr><td>&amp;yen=</td><td>&yen;=</td><td>&amp;yen=</td></tr>
+</table>
+<p>
+    This flag reverts back to pre-HTML Purifier 4.9.0 behavior.
+</p>
+--# vim: et sw=4 sts=4