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.AllowedComments.txt

@@ -0,0 +1,10 @@
+HTML.AllowedComments
+TYPE: lookup
+VERSION: 4.4.0
+DEFAULT: array()
+--DESCRIPTION--
+A whitelist which indicates what explicit comment bodies should be
+allowed, modulo leading and trailing whitespace.  See also %HTML.AllowedCommentsRegexp
+(these directives are union'ed together, so a comment is considered
+valid if any directive deems it valid.)
+--# vim: et sw=4 sts=4