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.DisableExternal.txt

@@ -0,0 +1,11 @@
+URI.DisableExternal
+TYPE: bool
+VERSION: 1.2.0
+DEFAULT: false
+--DESCRIPTION--
+Disables links to external websites.  This is a highly effective anti-spam
+and anti-pagerank-leech measure, but comes at a hefty price: nolinks or
+images outside of your domain will be allowed.  Non-linkified URIs will
+still be preserved.  If you want to be able to link to subdomains or use
+absolute URIs, specify %URI.Host for your website.
+--# vim: et sw=4 sts=4