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.SafeIframe.txt

@@ -0,0 +1,13 @@
+HTML.SafeIframe
+TYPE: bool
+VERSION: 4.4.0
+DEFAULT: false
+--DESCRIPTION--
+<p>
+    Whether or not to permit iframe tags in untrusted documents.  This
+    directive must be accompanied by a whitelist of permitted iframes,
+    such as %URI.SafeIframeRegexp, otherwise it will fatally error.
+    This directive has no effect on strict doctypes, as iframes are not
+    valid.
+</p>
+--# vim: et sw=4 sts=4