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.AllowedModules.txt

@@ -0,0 +1,20 @@
+HTML.AllowedModules
+TYPE: lookup/null
+VERSION: 2.0.0
+DEFAULT: NULL
+--DESCRIPTION--
+
+<p>
+    A doctype comes with a set of usual modules to use. Without having
+    to mucking about with the doctypes, you can quickly activate or
+    disable these modules by specifying which modules you wish to allow
+    with this directive. This is most useful for unit testing specific
+    modules, although end users may find it useful for their own ends.
+</p>
+<p>
+    If you specify a module that does not exist, the manager will silently
+    fail to use it, so be careful! User-defined modules are not affected
+    by this directive. Modules defined in %HTML.CoreModules are not
+    affected by this directive.
+</p>
+--# vim: et sw=4 sts=4