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.DefinitionRev.txt

@@ -0,0 +1,16 @@
+HTML.DefinitionRev
+TYPE: int
+VERSION: 2.0.0
+DEFAULT: 1
+--DESCRIPTION--
+
+<p>
+    Revision identifier for your custom definition specified in
+    %HTML.DefinitionID.  This serves the same purpose: uniquely identifying
+    your custom definition, but this one does so in a chronological
+    context: revision 3 is more up-to-date then revision 2.  Thus, when
+    this gets incremented, the cache handling is smart enough to clean
+    up any older revisions of your definition as well as flush the
+    cache.
+</p>
+--# vim: et sw=4 sts=4