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/Arborize.php

@@ -0,0 +1,71 @@
+<?php
+
+/**
+ * Converts a stream of HTMLPurifier_Token into an HTMLPurifier_Node,
+ * and back again.
+ *
+ * @note This transformation is not an equivalence.  We mutate the input
+ * token stream to make it so; see all [MUT] markers in code.
+ */
+class HTMLPurifier_Arborize
+{
+    public static function arborize($tokens, $config, $context) {
+        $definition = $config->getHTMLDefinition();
+        $parent = new HTMLPurifier_Token_Start($definition->info_parent);
+        $stack = array($parent->toNode());
+        foreach ($tokens as $token) {
+            $token->skip = null; // [MUT]
+            $token->carryover = null; // [MUT]
+            if ($token instanceof HTMLPurifier_Token_End) {
+                $token->start = null; // [MUT]
+                $r = array_pop($stack);
+                assert($r->name === $token->name);
+                assert(empty($token->attr));
+                $r->endCol = $token->col;
+                $r->endLine = $token->line;
+                $r->endArmor = $token->armor;
+                continue;
+            }
+            $node = $token->toNode();
+            $stack[count($stack)-1]->children[] = $node;
+            if ($token instanceof HTMLPurifier_Token_Start) {
+                $stack[] = $node;
+            }
+        }
+        assert(count($stack) == 1);
+        return $stack[0];
+    }
+
+    public static function flatten($node, $config, $context) {
+        $level = 0;
+        $nodes = array($level => new HTMLPurifier_Queue(array($node)));
+        $closingTokens = array();
+        $tokens = array();
+        do {
+            while (!$nodes[$level]->isEmpty()) {
+                $node = $nodes[$level]->shift(); // FIFO
+                list($start, $end) = $node->toTokenPair();
+                if ($level > 0) {
+                    $tokens[] = $start;
+                }
+                if ($end !== NULL) {
+                    $closingTokens[$level][] = $end;
+                }
+                if ($node instanceof HTMLPurifier_Node_Element) {
+                    $level++;
+                    $nodes[$level] = new HTMLPurifier_Queue();
+                    foreach ($node->children as $childNode) {
+                        $nodes[$level]->push($childNode);
+                    }
+                }
+            }
+            $level--;
+            if ($level && isset($closingTokens[$level])) {
+                while ($token = array_pop($closingTokens[$level])) {
+                    $tokens[] = $token;
+                }
+            }
+        } while ($level > 0);
+        return $tokens;
+    }
+}