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.SafeEmbed.txt

@@ -0,0 +1,13 @@
+HTML.SafeEmbed
+TYPE: bool
+VERSION: 3.1.1
+DEFAULT: false
+--DESCRIPTION--
+<p>
+    Whether or not to permit embed tags in documents, with a number of extra
+    security features added to prevent script execution. This is similar to
+    what websites like MySpace do to embed tags. Embed is a proprietary
+    element and will cause your website to stop validating; you should
+    see if you can use %Output.FlashCompat with %HTML.SafeObject instead
+    first.</p>
+--# vim: et sw=4 sts=4