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.SafeIframe.txt

@@ -0,0 +1,13 @@
+HTML.SafeIframe
+TYPE: bool
+VERSION: 4.4.0
+DEFAULT: false
+--DESCRIPTION--
+<p>
+    Whether or not to permit iframe tags in untrusted documents.  This
+    directive must be accompanied by a whitelist of permitted iframes,
+    such as %URI.SafeIframeRegexp, otherwise it will fatally error.
+    This directive has no effect on strict doctypes, as iframes are not
+    valid.
+</p>
+--# vim: et sw=4 sts=4