Add website to client repository

Closes #1567

The main reason I'm not simply merging Annika's PR is because this
way makes it clearer that I'm taking responsibility for all this
code, that it's mostly code I wrote, and also because it makes it
easier to ensure that none of the files have been changed.

(Not that I don't personally trust Annika, but I have something
resembling an obligation to users not to expose them to risks based
on personal trust.)

website/lib/htmlpurifier/HTMLPurifier/ConfigSchema/schema/HTML.XHTML.txt

@@ -0,0 +1,11 @@
+HTML.XHTML
+TYPE: bool
+DEFAULT: true
+VERSION: 1.1.0
+DEPRECATED-VERSION: 1.7.0
+DEPRECATED-USE: HTML.Doctype
+--DESCRIPTION--
+Determines whether or not output is XHTML 1.0 or HTML 4.01 flavor.
+--ALIASES--
+Core.XHTML
+--# vim: et sw=4 sts=4