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/Output.FixInnerHTML.txt

@@ -0,0 +1,15 @@
+Output.FixInnerHTML
+TYPE: bool
+VERSION: 4.3.0
+DEFAULT: true
+--DESCRIPTION--
+<p>
+  If true, HTML Purifier will protect against Internet Explorer's
+  mishandling of the <code>innerHTML</code> attribute by appending
+  a space to any attribute that does not contain angled brackets, spaces
+  or quotes, but contains a backtick.  This slightly changes the
+  semantics of any given attribute, so if this is unacceptable and
+  you do not use <code>innerHTML</code> on any of your pages, you can
+  turn this directive off.
+</p>
+--# vim: et sw=4 sts=4