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.DefinitionID.txt

@@ -0,0 +1,33 @@
+HTML.DefinitionID
+TYPE: string/null
+DEFAULT: NULL
+VERSION: 2.0.0
+--DESCRIPTION--
+
+<p>
+    Unique identifier for a custom-built HTML definition. If you edit
+    the raw version of the HTMLDefinition, introducing changes that the
+    configuration object does not reflect, you must specify this variable.
+    If you change your custom edits, you should change this directive, or
+    clear your cache. Example:
+</p>
+<pre>
+$config = HTMLPurifier_Config::createDefault();
+$config->set('HTML', 'DefinitionID', '1');
+$def = $config->getHTMLDefinition();
+$def->addAttribute('a', 'tabindex', 'Number');
+</pre>
+<p>
+    In the above example, the configuration is still at the defaults, but
+    using the advanced API, an extra attribute has been added. The
+    configuration object normally has no way of knowing that this change
+    has taken place, so it needs an extra directive: %HTML.DefinitionID.
+    If someone else attempts to use the default configuration, these two
+    pieces of code will not clobber each other in the cache, since one has
+    an extra directive attached to it.
+</p>
+<p>
+    You <em>must</em> specify a value to this directive to use the
+    advanced API features.
+</p>
+--# vim: et sw=4 sts=4