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/Cache.DefinitionImpl.txt

@@ -0,0 +1,14 @@
+Cache.DefinitionImpl
+TYPE: string/null
+VERSION: 2.0.0
+DEFAULT: 'Serializer'
+--DESCRIPTION--
+
+This directive defines which method to use when caching definitions,
+the complex data-type that makes HTML Purifier tick. Set to null
+to disable caching (not recommended, as you will see a definite
+performance degradation).
+
+--ALIASES--
+Core.DefinitionCache
+--# vim: et sw=4 sts=4