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.ForbiddenElements.txt

@@ -0,0 +1,20 @@
+HTML.ForbiddenElements
+TYPE: lookup
+VERSION: 3.1.0
+DEFAULT: array()
+--DESCRIPTION--
+<p>
+    This was, perhaps, the most requested feature ever in HTML
+    Purifier. Please don't abuse it! This is the logical inverse of
+    %HTML.AllowedElements, and it will override that directive, or any
+    other directive.
+</p>
+<p>
+    If possible, %HTML.Allowed is recommended over this directive, because it
+    can sometimes be difficult to tell whether or not you've forbidden all of
+    the behavior you would like to disallow. If you forbid <code>img</code>
+    with the expectation of preventing images on your site, you'll be in for
+    a nasty surprise when people start using the <code>background-image</code>
+    CSS property.
+</p>
+--# vim: et sw=4 sts=4