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/URI.MakeAbsolute.txt

@@ -0,0 +1,13 @@
+URI.MakeAbsolute
+TYPE: bool
+VERSION: 2.1.0
+DEFAULT: false
+--DESCRIPTION--
+
+<p>
+    Converts all URIs into absolute forms. This is useful when the HTML
+    being filtered assumes a specific base path, but will actually be
+    viewed in a different context (and setting an alternate base URI is
+    not possible). %URI.Base must be set for this directive to work.
+</p>
+--# vim: et sw=4 sts=4