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.SafeScripting.txt

@@ -0,0 +1,10 @@
+HTML.SafeScripting
+TYPE: lookup
+VERSION: 4.5.0
+DEFAULT: array()
+--DESCRIPTION--
+<p>
+    Whether or not to permit script tags to external scripts in documents.
+    Inline scripting is not allowed, and the script must match an explicit whitelist.
+</p>
+--# vim: et sw=4 sts=4