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/Attr.DefaultInvalidImage.txt

@@ -0,0 +1,9 @@
+Attr.DefaultInvalidImage
+TYPE: string
+DEFAULT: ''
+--DESCRIPTION--
+This is the default image an img tag will be pointed to if it does not have
+a valid src attribute.  In future versions, we may allow the image tag to
+be removed completely, but due to design issues, this is not possible right
+now.
+--# vim: et sw=4 sts=4