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/CSS.DefinitionRev.txt

@@ -0,0 +1,11 @@
+CSS.DefinitionRev
+TYPE: int
+VERSION: 2.0.0
+DEFAULT: 1
+--DESCRIPTION--
+
+<p>
+    Revision identifier for your custom definition. See
+    %HTML.DefinitionRev for details.
+</p>
+--# vim: et sw=4 sts=4