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.TidyLevel.txt

@@ -0,0 +1,24 @@
+HTML.TidyLevel
+TYPE: string
+VERSION: 2.0.0
+DEFAULT: 'medium'
+--DESCRIPTION--
+
+<p>General level of cleanliness the Tidy module should enforce.
+There are four allowed values:</p>
+<dl>
+    <dt>none</dt>
+    <dd>No extra tidying should be done</dd>
+    <dt>light</dt>
+    <dd>Only fix elements that would be discarded otherwise due to
+    lack of support in doctype</dd>
+    <dt>medium</dt>
+    <dd>Enforce best practices</dd>
+    <dt>heavy</dt>
+    <dd>Transform all deprecated elements and attributes to standards
+    compliant equivalents</dd>
+</dl>
+
+--ALLOWED--
+'none', 'light', 'medium', 'heavy'
+--# vim: et sw=4 sts=4