Replace existing autoloader with Composer-supplied

- Move Friendica namespace to `src`
- Move required `ezyang/htmlpurifier` to vendor
- Remove existing static autoloader
- Change boot.php reference to the autoloader

vendor/ezyang/htmlpurifier/library/HTMLPurifier/ConfigSchema/schema/HTML.AllowedElements.txt

@@ -0,0 +1,23 @@
+HTML.AllowedElements
+TYPE: lookup/null
+VERSION: 1.3.0
+DEFAULT: NULL
+--DESCRIPTION--
+<p>
+    If HTML Purifier's tag set is unsatisfactory for your needs, you can
+    overload it with your own list of tags to allow.  If you change
+    this, you probably also want to change %HTML.AllowedAttributes; see
+    also %HTML.Allowed which lets you set allowed elements and
+    attributes at the same time.
+</p>
+<p>
+    If you attempt to allow an element that HTML Purifier does not know
+    about, HTML Purifier will raise an error.  You will need to manually
+    tell HTML Purifier about this element by using the
+    <a href="http://htmlpurifier.org/docs/enduser-customize.html">advanced customization features.</a>
+</p>
+<p>
+    <strong>Warning:</strong> If another directive conflicts with the
+    elements here, <em>that</em> directive will win and override.
+</p>
+--# vim: et sw=4 sts=4