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/AttrTransform/Background.php

@@ -0,0 +1,28 @@
+<?php
+
+/**
+ * Pre-transform that changes proprietary background attribute to CSS.
+ */
+class HTMLPurifier_AttrTransform_Background extends HTMLPurifier_AttrTransform
+{
+    /**
+     * @param array $attr
+     * @param HTMLPurifier_Config $config
+     * @param HTMLPurifier_Context $context
+     * @return array
+     */
+    public function transform($attr, $config, $context)
+    {
+        if (!isset($attr['background'])) {
+            return $attr;
+        }
+
+        $background = $this->confiscateAttr($attr, 'background');
+        // some validation should happen here
+
+        $this->prependCSS($attr, "background-image:url($background);");
+        return $attr;
+    }
+}
+
+// vim: et sw=4 sts=4