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.TidyAdd.txt

@@ -0,0 +1,8 @@
+HTML.TidyAdd
+TYPE: lookup
+VERSION: 2.0.0
+DEFAULT: array()
+--DESCRIPTION--
+
+Fixes to add to the default set of Tidy fixes as per your level.
+--# vim: et sw=4 sts=4